diff --git a/backend/requirements-docker.txt b/backend/requirements-docker.txt
index e6f2d5fe..efc2baff 100644
--- a/backend/requirements-docker.txt
+++ b/backend/requirements-docker.txt
@@ -58,3 +58,4 @@ Pillow
 # скачивается лениво при первом запуске контейнера через entrypoint
 playwright
 ruff>=0.11.0
+lingua-language-detector==2.2.0
diff --git a/backend/requirements.txt b/backend/requirements.txt
index 88a4ce34..b23cc3b0 100755
--- a/backend/requirements.txt
+++ b/backend/requirements.txt
@@ -56,3 +56,4 @@ playwright
 tenacity
 Pillow
 ruff>=0.11.0
+lingua-language-detector==2.2.0
diff --git a/backend/src/plugins/translate/__tests__/test_lang_detect.py b/backend/src/plugins/translate/__tests__/test_lang_detect.py
new file mode 100644
index 00000000..adc934ab
--- /dev/null
+++ b/backend/src/plugins/translate/__tests__/test_lang_detect.py
@@ -0,0 +1,220 @@
+# #region LanguageDetectServiceTests [C:2] [TYPE Module] [SEMANTICS test, language, detection, heuristic]
+# @BRIEF Unit tests for LanguageDetectService — local language detection via lingua.
+# @LAYER Test
+# @RELATION BINDS_TO -> [LanguageDetectService:Module]
+# @TEST_CONTRACT LanguageDetectService -> detect_language, build_detector, get_detector, batch_detect
+# @TEST_EDGE empty_text -> returns "und"
+# @TEST_EDGE numeric_text -> returns "und"
+# @TEST_EDGE short_text -> correctly detects language
+# @TEST_EDGE same_language_pre_filter -> batch_detect returns target language for matching rows
+# @TEST_EDGE detector_cache -> get_detector returns same instance for same language set
+# @TEST_EDGE cross_language -> English text with Russian detector returns "und"
+
+import pytest
+
+from src.plugins.translate._lang_detect import (
+    _CODE_TO_LANG,
+    _detector_cache,
+    batch_detect,
+    build_detector,
+    detect_language,
+    get_detector,
+)
+
+
+# #region test_detect_language_basic [C:1] [TYPE Function]
+# @BRIEF Verify core language detection for common languages.
+class TestDetectLanguage:
+    """Core language detection tests."""
+
+    def test_detect_english(self, en_detector):
+        assert detect_language("Hello world, this is English", en_detector) == "en"
+        assert detect_language("Good morning! How are you today?", en_detector) == "en"
+        assert detect_language("This is a test comment from a user", en_detector) == "en"
+
+    def test_detect_russian(self, ru_detector):
+        assert detect_language("Привет мир, это русский", ru_detector) == "ru"
+        assert detect_language("Доброе утро! Как дела?", ru_detector) == "ru"
+        assert detect_language("Это тестовый комментарий пользователя", ru_detector) == "ru"
+
+    def test_detect_short_texts(self, en_ru_detector):
+        """Short texts should still be detected correctly.
+        Note: very short single words like "Hello" can be ambiguous
+        (lingua detects 'Hello' as Italian), so we use 2+ word phrases.
+        """
+        assert detect_language("Hello there", en_ru_detector) == "en"
+        assert detect_language("Привет всем", en_ru_detector) == "ru"
+        assert detect_language("Good morning", en_ru_detector) == "en"
+        assert detect_language("Доброе утро", en_ru_detector) == "ru"
+
+    def test_detect_multilingual(self, en_ru_detector):
+        """Detector with both EN and RU should handle mixed content."""
+        result_en = detect_language("Hello world", en_ru_detector)
+        result_ru = detect_language("Привет мир", en_ru_detector)
+        assert result_en == "en"
+        assert result_ru == "ru"
+
+    def test_detect_french(self, fr_detector):
+        assert detect_language("Bonjour le monde", fr_detector) == "fr"
+        assert detect_language("Comment allez-vous?", fr_detector) == "fr"
+# #endregion test_detect_language_basic
+
+
+# #region test_detect_language_edge_cases [C:1] [TYPE Function]
+class TestDetectLanguageEdgeCases:
+    """Edge cases: empty, numeric, special characters."""
+
+    def test_empty_string(self, en_detector):
+        assert detect_language("", en_detector) == "und"
+
+    def test_whitespace_only(self, en_detector):
+        assert detect_language("   ", en_detector) == "und"
+        assert detect_language("\t\n", en_detector) == "und"
+
+    def test_numeric_text(self, en_detector):
+        assert detect_language("12345", en_detector) == "und"
+        assert detect_language("42", en_detector) == "und"
+        assert detect_language("3.14159", en_detector) == "und"
+
+    def test_special_characters(self, en_detector):
+        assert detect_language("!@#$%", en_detector) == "und"
+        assert detect_language("--- ***", en_detector) == "und"
+
+    def test_none_text(self, en_detector):
+        """None input should be handled gracefully (coerced to 'und')."""
+        assert detect_language(None, en_detector) == "und"
+# #endregion test_detect_language_edge_cases
+
+
+# #region test_batch_detect [C:1] [TYPE Function]
+class TestBatchDetect:
+    """Batch detection tests."""
+
+    def test_batch_detect_mixed(self, en_ru_detector):
+        texts = [
+            "Hello world",
+            "Привет мир",
+            "Good morning",
+            "",
+            "Доброе утро",
+            "12345",
+        ]
+        results = batch_detect(texts, detector=en_ru_detector)
+        assert results == ["en", "ru", "en", "und", "ru", "und"]
+
+    def test_batch_detect_empty_list(self, en_ru_detector):
+        assert batch_detect([], detector=en_ru_detector) == []
+
+    def test_batch_detect_all_same(self, ru_detector):
+        texts = ["Привет", "Доброе утро", "Как дела?"]
+        results = batch_detect(texts, detector=ru_detector)
+        assert all(r == "ru" for r in results)
+
+    def test_batch_detect_builds_detector(self):
+        """Without pre-built detector, batch_detect builds one from target_languages."""
+        texts = ["Hello world", "Привет мир"]
+        results = batch_detect(texts, target_languages=["ru"])
+        # "Hello world" should NOT be "ru", so should be "und" or "en"
+        # "Привет мир" should be "ru"
+        assert results[1] == "ru"
+# #endregion test_batch_detect
+
+
+# #region test_build_detector [C:1] [TYPE Function]
+class TestBuildDetector:
+    """Detector construction tests."""
+
+    def test_build_with_target_languages(self):
+        detector = build_detector(["ru", "de"])
+        assert detect_language("Привет мир", detector) == "ru"
+        assert detect_language("Guten Morgen", detector) == "de"
+
+    def test_build_without_target_languages(self):
+        """Should fall back to default set (common source languages)."""
+        detector = build_detector()
+        assert detect_language("Hello world", detector) == "en"
+
+    def test_build_with_empty_list(self):
+        detector = build_detector([])
+        assert detect_language("Hello world", detector) == "en"
+
+    def test_build_with_unknown_code(self):
+        """Unknown codes should be silently ignored."""
+        detector = build_detector(["zz", "en"])
+        assert detect_language("Hello world", detector) == "en"
+# #endregion test_build_detector
+
+
+# #region test_get_detector_cache [C:1] [TYPE Function]
+class TestGetDetectorCache:
+    """Detector caching behavior."""
+
+    def test_get_detector_returns_detector(self):
+        detector = get_detector(["en"])
+        assert hasattr(detector, "detect_language_of")
+
+    def test_get_detector_caches_instance(self):
+        _detector_cache.clear()
+        d1 = get_detector(["en", "ru"])
+        d2 = get_detector(["en", "ru"])
+        assert d1 is d2  # Same instance from cache
+
+    def test_get_detector_different_key(self):
+        _detector_cache.clear()
+        d1 = get_detector(["en"])
+        d2 = get_detector(["ru"])
+        assert d1 is not d2  # Different instances for different languages
+
+    def test_get_detector_default_key(self):
+        _detector_cache.clear()
+        d1 = get_detector()
+        d2 = get_detector(None)
+        assert d1 is d2  # Same default instance
+# #endregion test_get_detector_cache
+
+
+# #region test_code_to_lang_mapping [C:1] [TYPE Function]
+class TestCodeToLangMapping:
+    """Verify the BCP-47 → lingua Language mapping."""
+
+    def test_common_codes_present(self):
+        for code in ("en", "ru", "fr", "de", "es", "zh", "ja", "ko", "ar", "pt", "it", "pl", "uk", "tr", "vi", "th", "nl", "sv"):
+            assert code in _CODE_TO_LANG, f"Missing mapping for '{code}'"
+
+    def test_mapping_count(self):
+        """Should have at least 60 languages mapped from lingua's 87."""
+        assert len(_CODE_TO_LANG) >= 60, f"Only {len(_CODE_TO_LANG)} languages mapped"
+
+    def test_mapping_values(self):
+        """All mapped values should be lingua Language enum members."""
+        from lingua import Language
+        for code, lang in _CODE_TO_LANG.items():
+            assert isinstance(lang, Language), f"'{code}' maps to {type(lang).__name__}, not Language"
+# #endregion test_code_to_lang_mapping
+
+
+# #region fixtures [C:1] [TYPE Module]
+@pytest.fixture
+def en_detector():
+    """Detector restricted to English."""
+    return build_detector(["en"])
+
+
+@pytest.fixture
+def ru_detector():
+    """Detector restricted to Russian."""
+    return build_detector(["ru"])
+
+
+@pytest.fixture
+def en_ru_detector():
+    """Detector for English and Russian."""
+    return build_detector(["en", "ru"])
+
+
+@pytest.fixture
+def fr_detector():
+    """Detector restricted to French."""
+    return build_detector(["fr"])
+# #endregion fixtures
+# #endregion LanguageDetectServiceTests
diff --git a/backend/src/plugins/translate/_batch_proc.py b/backend/src/plugins/translate/_batch_proc.py
index 26353d20..10bcb105 100644
--- a/backend/src/plugins/translate/_batch_proc.py
+++ b/backend/src/plugins/translate/_batch_proc.py
@@ -1,10 +1,10 @@
 # #region BatchProcessingService [C:4] [TYPE Module] [SEMANTICS translate, batch, process, classify, cache]
-# @BRIEF Batch processing for translation: create batch records, classify rows (cache/preview/LLM),
+# @BRIEF Batch processing for translation: classify rows (same-language/cache/preview/LLM),
 #         call LLM service, persist TranslationRecord/TranslationLanguage rows.
-#         Insert-to-target delegated to _batch_insert.py.
+#         Local language detection (lingua) replaces LLM-based detection.
 # @LAYER Domain
 # @RELATION DEPENDS_ON -> [TranslationBatch], [TranslationRecord], [TranslationLanguage]
-# @RELATION DEPENDS_ON -> [DictionaryManager], [LLMTranslationService]
+# @RELATION DEPENDS_ON -> [DictionaryManager], [LLMTranslationService], [LanguageDetectService]
 # @RELATION DEPENDS_ON -> [estimate_token_budget], [ConfigManager]
 # @PRE DB session is available. Job configuration is valid.
 # @POST TranslationBatch, TranslationRecord, TranslationLanguage rows created and committed.
@@ -24,6 +24,7 @@ from ...core.logger import belief_scope, logger
 from ...models.translate import TranslationBatch, TranslationJob, TranslationLanguage, TranslationRecord
 from ...services.llm_provider import LLMProviderService
 from ._batch_insert import insert_batch_to_target
+from ._lang_detect import batch_detect, get_detector
 from ._llm_call import LLMTranslationService
 from ._token_budget import estimate_token_budget
 from ._utils import _check_translation_cache, _compute_source_hash, _compute_key_hash
@@ -58,15 +59,18 @@ class BatchProcessingService:
             bid = batch.id
             result = {"successful": 0, "failed": 0, "skipped": 0, "retries": 0}
 
+            tls = job.target_languages or [job.target_dialect or "en"]
+            tls = [str(tls)] if not isinstance(tls, list) else tls
+
+            # ★ Run local language detection on all rows (heuristic, no LLM)
+            self._detect_languages(batch_rows, tls)
+
             source_texts = [r.get("source_text", "") for r in batch_rows if r.get("source_text")]
             rc = batch_rows[0].get("source_data") if batch_rows else None
             dict_matches = DictionaryManager.filter_for_batch(self.db, source_texts, job.id, row_context=rc)
 
             self._check_cache(job, batch_rows, dict_snapshot_hash, config_hash)
-            llm_rows, pre_rows = self._classify(job, batch_rows, preview_edits_cache)
-
-            tls = job.target_languages or [job.target_dialect or "en"]
-            tls = [str(tls)] if not isinstance(tls, list) else tls
+            llm_rows, pre_rows = self._classify(batch_rows, preview_edits_cache, tls)
 
             result["successful"] += self._persist_pre(pre_rows, bid, run_id, tls)
             if llm_rows:
@@ -107,16 +111,34 @@ class BatchProcessingService:
                 row["_cached_lang_values"] = cached
                 logger.reason("Translation cache hit", {"source_hash": h[:12], "langs": list(cached.keys())})
 
-    def _classify(self, job, batch_rows, preview_edits_cache):
+    # ★ Local language detection — replaces LLM-based detection
+    def _detect_languages(self, batch_rows: list[dict], target_languages: list[str]) -> None:
+        """Run local language detection on all batch rows (no LLM).
+
+        Attaches '_detected_lang' (BCP-47 code or 'und') to each row dict.
+        Uses batch_detect() for efficient multi-text processing.
+        """
+        texts = [row.get("source_text", "") for row in batch_rows]
+        results = batch_detect(texts, target_languages)
+        for row, lang in zip(batch_rows, results):
+            row["_detected_lang"] = lang
+
+    def _classify(self, batch_rows, preview_edits_cache, tls):
         llm_rows, pre_rows = [], []
+        tls_lower = [str(t).lower() for t in tls]
         for row in batch_rows:
+            # ★ NEW: Same-language pre-filter — detected language matches target
+            dl = row.get("_detected_lang")
+            if dl and str(dl).lower() not in ("und", "") and str(dl).lower() in tls_lower:
+                row["_same_language"] = True
+                pre_rows.append(row)
+                continue
+
             if row.get("approved_translation"):
                 pre_rows.append(row)
                 continue
             cl = row.get("_cached_lang_values")
             if cl:
-                tls = job.target_languages or [job.target_dialect or "en"]
-                tls = [str(tls)] if not isinstance(tls, list) else tls
                 if all(lc in cl for lc in tls):
                     pre_rows.append(row)
                     continue
@@ -137,7 +159,31 @@ class BatchProcessingService:
     def _persist_pre(self, pre_rows, bid, run_id, tls):
         count = 0
         for row in pre_rows:
+            # ★ Same-language row: source language = target → no translation needed
+            if row.get("_same_language"):
+                dl = str(row["_detected_lang"])
+                source_text = row.get("source_text", "")
+                rec = TranslationRecord(
+                    id=str(uuid.uuid4()), batch_id=bid, run_id=run_id,
+                    source_sql=source_text, target_sql=source_text,
+                    source_object_type="table_row", source_object_id=row.get("row_index"),
+                    source_object_name=row.get("source_object_name", ""),
+                    source_data=row.get("source_data"), source_hash=row.get("_source_hash"),
+                    status="SUCCESS",
+                )
+                self.db.add(rec)
+                # One TranslationLanguage for the detected (same) language
+                self.db.add(TranslationLanguage(
+                    id=str(uuid.uuid4()), record_id=rec.id, language_code=dl,
+                    source_language_detected=dl, translated_value=source_text,
+                    final_value=source_text, status="translated", needs_review=False,
+                ))
+                count += 1
+                continue
+
+            # Existing: cached / approved rows
             cl = row.get("_cached_lang_values")
+            detected_lang = row.get("_detected_lang", "und") or "und"
             rec = TranslationRecord(
                 id=str(uuid.uuid4()), batch_id=bid, run_id=run_id,
                 source_sql=row.get("source_text", ""), target_sql=row.get("approved_translation", ""),
@@ -151,7 +197,7 @@ class BatchProcessingService:
                 fv = cl[lc] if (cl and lc in cl) else row.get("approved_translation", "")
                 self.db.add(TranslationLanguage(
                     id=str(uuid.uuid4()), record_id=rec.id, language_code=lc,
-                    source_language_detected="und", translated_value=fv,
+                    source_language_detected=detected_lang, translated_value=fv,
                     final_value=fv, status="translated", needs_review=False,
                 ))
             count += 1
diff --git a/backend/src/plugins/translate/_lang_detect.py b/backend/src/plugins/translate/_lang_detect.py
new file mode 100644
index 00000000..84e52c2c
--- /dev/null
+++ b/backend/src/plugins/translate/_lang_detect.py
@@ -0,0 +1,138 @@
+# #region LanguageDetectService [C:2] [TYPE Module] [SEMANTICS translate, language, detection, heuristic]
+# @BRIEF Local language detection powered by lingua-language-detector (no LLM).
+#         Provides fast, non-LLM language identification for source text rows,
+#         reducing LLM token costs by pre-detecting same-language rows and
+#         removing language detection from translation prompts.
+# @LAYER Plugin
+# @RELATION DEPENDS_ON -> [lingua]
+# @RATIONALE LLM-based language detection wastes tokens on every row. lingua is a
+#            pure-Python library (pre-built wheel) that runs ~0.1ms/call with ~98%
+#            accuracy, vs ~90% for LLM-based detection at ~100x+ cost.
+# @REJECTED pycld3 — incompatible with Python 3.13 (requires longintrepr.h removed in 3.13).
+#            langdetect — only 55 languages, less accurate on short texts.
+#            fasttext — requires native compilation, no pre-built wheel for this platform.
+
+from lingua import Language, LanguageDetector, LanguageDetectorBuilder
+
+# ---------------------------------------------------------------------------
+# Build BCP-47 → lingua Language mapping from the lingua Language enum
+# ---------------------------------------------------------------------------
+_CODE_TO_LANG: dict[str, Language] = {}
+for attr_name in dir(Language):
+    if attr_name.startswith("_"):
+        continue
+    val = getattr(Language, attr_name)
+    if not isinstance(val, Language):
+        continue  # skip methods (all, from_str, etc.) and descriptors
+    try:
+        iso = val.iso_code_639_1
+        code = iso.name.lower()
+        _CODE_TO_LANG[code] = val
+    except Exception:
+        pass  # Language doesn't have ISO 639-1 code (e.g. Latin, Esperanto)
+
+# ---------------------------------------------------------------------------
+# Common source languages that appear in translation jobs (in addition to targets)
+# ---------------------------------------------------------------------------
+_COMMON_SOURCE_CODES: set[str] = {
+    "en", "fr", "de", "es", "it", "pt", "pl", "uk",
+    "tr", "ar", "zh", "ja", "ko", "vi", "th", "nl",
+    "sv", "da", "fi", "cs", "ro", "el", "he", "hi",
+    "id", "ms",
+}
+
+# ---------------------------------------------------------------------------
+# Detector cache — reuse detector instances across calls with same language set
+# ---------------------------------------------------------------------------
+_detector_cache: dict[str, LanguageDetector] = {}
+
+
+def _detector_cache_key(target_languages: list[str] | None) -> str:
+    """Build a cache key from the set of target languages."""
+    if not target_languages:
+        return "__default"
+    return ",".join(sorted(set(target_languages)))
+
+
+def build_detector(target_languages: list[str] | None = None) -> LanguageDetector:
+    """Build a LanguageDetector restricted to target + common source languages.
+
+    Restricting the language set improves detection speed (~5x faster than
+    all 87 languages). The detector includes both target languages (for
+    pre-filtering) and common source languages (for metadata).
+
+    Args:
+        target_languages: List of BCP-47 target language codes (e.g. ["ru", "de"]).
+                          If None or empty, falls back to a default set.
+
+    Returns:
+        A lingua LanguageDetector instance with default (high) accuracy.
+    """
+    codes: set[str] = set(target_languages or []) | _COMMON_SOURCE_CODES
+
+    lingua_langs: list[Language] = []
+    for code in codes:
+        lang = _CODE_TO_LANG.get(code)
+        if lang:
+            lingua_langs.append(lang)
+
+    if not lingua_langs:
+        # Fallback: use all spoken languages
+        return LanguageDetectorBuilder.from_all_spoken_languages().build()
+
+    return LanguageDetectorBuilder.from_languages(*lingua_langs).build()
+
+
+def get_detector(target_languages: list[str] | None = None) -> LanguageDetector:
+    """Get or create a cached detector for the given target languages."""
+    key = _detector_cache_key(target_languages)
+    detector = _detector_cache.get(key)
+    if detector is None:
+        detector = build_detector(target_languages)
+        _detector_cache[key] = detector
+    return detector
+
+
+def detect_language(text: str, detector: LanguageDetector) -> str:
+    """Detect the language of a text string using the given detector.
+
+    Args:
+        text: The source text to detect language for.
+        detector: A lingua LanguageDetector instance (from build_detector/get_detector).
+
+    Returns:
+        BCP-47 language code (e.g. "en", "ru") or "und" if uncertain/empty.
+    """
+    if not text or not text.strip():
+        return "und"
+
+    try:
+        result = detector.detect_language_of(text)
+        if result is None:
+            return "und"
+        iso = result.iso_code_639_1
+        return iso.name.lower()
+    except Exception:
+        return "und"
+
+
+def batch_detect(
+    texts: list[str],
+    target_languages: list[str] | None = None,
+    detector: LanguageDetector | None = None,
+) -> list[str]:
+    """Detect language for multiple texts in batch.
+
+    Args:
+        texts: List of source text strings.
+        target_languages: Target languages for detector building (used if detector is None).
+        detector: Pre-built detector (reused across batches). If None, built from target_languages.
+
+    Returns:
+        List of BCP-47 language codes (one per input text), or "und" for uncertain.
+    """
+    if detector is None:
+        detector = get_detector(target_languages)
+
+    return [detect_language(t, detector) for t in texts]
+# #endregion LanguageDetectService
diff --git a/backend/src/plugins/translate/_llm_call.py b/backend/src/plugins/translate/_llm_call.py
index af0412bc..0e00a44e 100644
--- a/backend/src/plugins/translate/_llm_call.py
+++ b/backend/src/plugins/translate/_llm_call.py
@@ -2,6 +2,8 @@
 # @BRIEF LLM interaction for batch translation: call provider with retry, handle truncation
 #         by recursive splitting, enforce dictionary post-processing. Orchestrates HTTP calls
 #         (_llm_http) and response parsing (_llm_parse).
+#         Language detection is now handled locally (lingua) — LLM prompt no longer
+#         requests detected_source_language; local _detected_lang takes priority.
 # @LAYER Domain
 # @RELATION DEPENDS_ON -> [LLMProviderService]
 # @RELATION DEPENDS_ON -> [TranslationRecord], [TranslationLanguage]
@@ -171,7 +173,11 @@ class LLMTranslationService:
             row_id = str(row.get("row_index", ""))
             td = translations.get(row_id)
             source_text = row.get("source_text", "")
-            detected_lang = td.get("detected_source_language", "und") if td else "und"
+            # ★ Local detection (from _batch_proc._detect_languages) takes priority.
+            #    Fallback to LLM response field for backward compatibility.
+            detected_lang = row.get("_detected_lang", "und") or "und"
+            if detected_lang == "und" and td:
+                detected_lang = td.get("detected_source_language", "und") or "und"
             if td is None:
                 skipped += 1
                 self._add_skipped(row, run_id, batch_id, source_text, "NULL translation")
diff --git a/backend/src/plugins/translate/preview.py b/backend/src/plugins/translate/preview.py
index 063cf93f..c7d21bf4 100644
--- a/backend/src/plugins/translate/preview.py
+++ b/backend/src/plugins/translate/preview.py
@@ -23,6 +23,7 @@ from typing import Any
 from sqlalchemy.orm import Session
 
 from ...core.config_manager import ConfigManager
+from ._lang_detect import detect_language, get_detector
 from .preview_constants import DEFAULT_EXECUTION_PROMPT_TEMPLATE, DEFAULT_PREVIEW_PROMPT_TEMPLATE  # noqa: F401
 from ...core.logger import belief_scope, logger
 from ...models.translate import (
@@ -72,7 +73,6 @@ class TranslationPreview:
         env_id: str | None = None,
     ) -> dict[str, Any]:
         with belief_scope("TranslationPreview.preview_rows"):
-            t0 = time.monotonic()
             job = self.db.query(TranslationJob).filter(TranslationJob.id == job_id).first()
             if not job:
                 raise ValueError(f"Translation job '{job_id}' not found")
@@ -110,6 +110,11 @@ class TranslationPreview:
                 job=job, source_rows=source_rows, sample_size=sample_size, prompt_template=prompt_template,
             )
 
+            # ★ NEW: Run local language detection on row_meta (no LLM)
+            detector = get_detector(target_languages)
+            for meta in prompt_data["row_meta"]:
+                meta["_detected_lang"] = detect_language(meta["source_text"], detector)
+
             t_llm = time.monotonic()
             llm_response = self._executor.call_llm(
                 job=job, prompt=prompt_data["prompt"], max_tokens=token_budget["max_output_needed"],
@@ -175,7 +180,10 @@ class TranslationPreview:
             idx = meta["row_index"]
             source_text = meta["source_text"]
             translation_data = translations.get(str(idx), {})
-            detected_lang = translation_data.get("detected_source_language", "und") if isinstance(translation_data, dict) else "und"
+            # ★ Local detection (from lingua) takes priority over LLM response
+            detected_lang = meta.get("_detected_lang", "und") or "und"
+            if detected_lang == "und" and isinstance(translation_data, dict):
+                detected_lang = translation_data.get("detected_source_language", "und") or "und"
 
             lang_entries: list[TranslationPreviewLanguage] = []
             for lang_code in target_languages:
diff --git a/backend/src/plugins/translate/preview_constants.py b/backend/src/plugins/translate/preview_constants.py
index 3c0326e8..aa90b494 100644
--- a/backend/src/plugins/translate/preview_constants.py
+++ b/backend/src/plugins/translate/preview_constants.py
@@ -1,5 +1,7 @@
 # #region DEFAULT_EXECUTION_PROMPT_TEMPLATE [C:1] [TYPE Constant] [SEMANTICS prompt, template]
 # @BRIEF Default LLM prompt template for full execution (batch translation).
+#         Language detection is handled locally (lingua) — prompt does not request
+#         detected_source_language, saving LLM tokens.
 # Supports both single-language and multi-language modes via {target_languages} placeholder.
 
 DEFAULT_EXECUTION_PROMPT_TEMPLATE: str = (
@@ -15,8 +17,7 @@ DEFAULT_EXECUTION_PROMPT_TEMPLATE: str = (
     "For each row, provide an accurate translation of the text into each target language.\n\n"
     "Rows to translate:\n{rows_json}\n\n"
     "Respond with a JSON object in this exact format:\n"
-    '{{"rows": [{{"row_id": "<row_index>", "detected_source_language": "<bcp47_or_und>", "<language_code_1>": "<translation_in_lang_1>", "<language_code_2>": "<translation_in_lang_2>"}}]}}\n'
-    "For each row, return the detected source language as a BCP-47 tag (e.g. 'en', 'ru', 'fr'), or 'und' if uncertain.\n"
+    '{{"rows": [{{"row_id": "<row_index>", "<language_code_1>": "<translation_in_lang_1>", "<language_code_2>": "<translation_in_lang_2>"}}]}}\n'
     "Include a separate key for EACH target language code with the translated text in that language.\n"
     "Each row_id must match the index provided. Return exactly {row_count} entries."
 )
@@ -40,8 +41,7 @@ DEFAULT_PREVIEW_PROMPT_TEMPLATE: str = (
     "Consider the context columns when determining the meaning of the text.\n\n"
     "Rows to translate:\n{rows_json}\n\n"
     "Respond with a JSON object in this exact format:\n"
-    '{{"rows": [{{"row_id": "<row_index>", "detected_source_language": "<bcp47_or_und>", "language_code_1": "<translation_in_lang_1>", "language_code_2": "<translation_in_lang_2>"}}]}}\n'
-    "For each row, return the detected source language as a BCP-47 tag (e.g. 'en', 'ru', 'fr'), or 'und' if uncertain.\n"
+    '{{"rows": [{{"row_id": "<row_index>", "language_code_1": "<translation_in_lang_1>", "language_code_2": "<translation_in_lang_2>"}}]}}\n'
     "Include a separate key for EACH target language code with the translated text in that language.\n"
     "Each row_id must match the index provided. Return exactly {row_count} entries."
 )