fix(mistral): honor dataset language in figure prompts (#18021)

### Summary

Refs #17885.

Mistral figure enrichment now receives the dataset language through the
production parsing path. `by_mistral_ocr` forwards `lang` to
`MistralParser.parse_pdf`; the parser stores the normalized language and
passes it to the figure-description prompt. Empty or missing values
still fall back to English.

deepdoc/parser/mistral_parser.py

@@ -78,6 +78,7 @@ class MistralParser(RAGFlowPdfParser):
         # Optional tenant vision model (LLMBundle) for figure description;
         # set from parse_pdf's vision_model kwarg. None -> no enrichment.
         self.vision_model = None
+        self.language = "English"
 
     # ------------------------------------------------------------------
     # Page image rendering
@@ -326,7 +327,8 @@ class MistralParser(RAGFlowPdfParser):
 
             # vision_llm_chunk expects a PIL Image (it calls img.size / img.save),
             # not raw bytes — pass the crop directly.
-            desc = vision_llm_chunk(binary=img, vision_model=self.vision_model, prompt=vision_llm_figure_describe_prompt())
+            self.logger.debug("[Mistral OCR] describing figure with language=%s", self.language)
+            desc = vision_llm_chunk(binary=img, vision_model=self.vision_model, prompt=vision_llm_figure_describe_prompt(language=self.language))
             return (desc or "").strip()
         except Exception:
             self.logger.info("[Mistral OCR] figure description skipped", exc_info=True)
@@ -457,6 +459,7 @@ class MistralParser(RAGFlowPdfParser):
     def parse_pdf(self, filepath: str | PathLike[str], binary=None, callback=None, parse_method: str = "raw", from_page: int = 0, to_page: int = MAXIMUM_PAGE_NUMBER, **kwargs) -> tuple[list, list]:
         # Optional tenant vision model for figure description (best-effort).
         self.vision_model = kwargs.pop("vision_model", None)
+        self.language = kwargs.pop("lang", None) or "English"
 
         # Load bytes.
         if binary is not None:

rag/app/naive.py

@@ -396,6 +396,7 @@ def by_mistral_ocr(
                     parse_method=parse_method,
                     from_page=from_page,
                     to_page=to_page,
+                    lang=lang,
                     **kwargs,
                 )
                 return sections, tables, pdf_parser

test/unit_test/deepdoc/parser/test_mistral_parser.py

@@ -539,8 +539,11 @@ def test_parse_pdf_consumes_vision_model_kwarg(monkeypatch, tmp_path):
     _patch_render(m, p, 2)
     pdf = tmp_path / "x.pdf"
     pdf.write_bytes(b"%PDF-1.4 minimal")
-    p.parse_pdf(str(pdf), vision_model="VM")
+    p.parse_pdf(str(pdf), vision_model="VM", lang="Japanese")
     assert p.vision_model == "VM"  # popped from kwargs into self, not forwarded to _call_ocr
+    assert p.language == "Japanese"
+    p.parse_pdf(str(pdf), vision_model="VM", lang="")
+    assert p.language == "English"
 
 
 def test_describe_image_passes_pil_image_not_bytes(monkeypatch):
@@ -562,10 +565,11 @@ def test_describe_image_passes_pil_image_not_bytes(monkeypatch):
     pic = ModuleType("rag.app.picture")
     pic.vision_llm_chunk = lambda binary, vision_model, prompt=None, callback=None: (captured.update(kind=type(binary).__name__), "a white square")[1]
     gen = ModuleType("rag.prompts.generator")
-    gen.vision_llm_figure_describe_prompt = lambda: "describe"
+    gen.vision_llm_figure_describe_prompt = lambda language: (captured.update(language=language), "describe")[1]
     monkeypatch.setitem(_sys.modules, "rag.app.picture", pic)
     monkeypatch.setitem(_sys.modules, "rag.prompts.generator", gen)
 
     out = p._describe_image("@@1\t0\t0\t40\t40##")
     assert out == "a white square"
     assert captured["kind"] == "Image"  # PIL Image, not 'bytes'
+    assert captured["language"] == "English"

test/unit_test/rag/app/test_vision_language_callers.py

@@ -98,3 +98,30 @@ def test_markdown_chunk_forwards_language_to_model_and_figure_parser(monkeypatch
     assert parser_factory.call_args.kwargs["vision_model"] is vision_model
     assert parser_factory.call_args.kwargs["lang"] == "Japanese"
     parser_instance.assert_called_once()
+
+
+@pytest.mark.p1
+def test_mistral_ocr_forwards_language_to_parser(monkeypatch):
+    parser = Mock()
+    parser.parse_pdf.return_value = (["section"], [])
+    ocr_model = Mock(mdl=parser)
+    monkeypatch.setattr(naive, "resolve_model_config", Mock(return_value={"llm_name": "mistral-ocr"}))
+    monkeypatch.setattr(naive, "LLMBundle", Mock(return_value=ocr_model))
+
+    sections, tables, returned_parser = naive.by_mistral_ocr(
+        "document.pdf",
+        binary=b"pdf",
+        from_page=2,
+        to_page=5,
+        lang="Japanese",
+        callback=lambda *_args, **_kwargs: None,
+        parse_method="raw",
+        mistral_ocr_llm_name="mistral-ocr",
+        tenant_id="tenant-id",
+        vision_model=object(),
+    )
+
+    assert sections == ["section"]
+    assert tables == []
+    assert returned_parser is parser
+    assert parser.parse_pdf.call_args.kwargs["lang"] == "Japanese"