feat: add native Dutch language support for BM25 tokenization (#14140)

## Summary
- Add language-aware Snowball stemmer to `RagTokenizer` supporting 16
languages (Dutch, German, French, Spanish, etc.)
- Thread the KB `language` parameter through the full tokenization
pipeline (14 parser modules + task executor)
- Add Dutch to the frontend language lists and cross-language form

## Problem
RAGFlow uses the English Porter stemmer + WordNet lemmatizer for **all**
BM25 tokenization, regardless of the knowledge base language setting.
This produces incorrect stems for non-English text. For example:

| Dutch word | Dutch stemmer | English Porter |
|---|---|---|
| documenten | document | documenten (unchanged!) |
| gebruikers | gebruiker | gebruik (over-stemmed) |
| instellingen | instell | instellingen (unchanged!) |

This degrades BM25 recall for any non-English knowledge base.

## Solution
NLTK already ships Snowball stemmers for 16 languages. This PR:

1. **`rag/nlp/rag_tokenizer.py`**: Overrides `tokenize()` with
`set_language()` and `_normalize_token()` that selects the correct NLTK
Snowball stemmer. Falls back to Porter for unmapped languages (Chinese,
Japanese, Korean, etc. — these use character-based tokenization anyway).
2. **`rag/nlp/__init__.py`** + **14 `rag/app/*.py` parsers** +
**`rag/svr/task_executor.py`**: Threads the `language` parameter through
`tokenize()`, `tokenize_chunks()`, `tokenize_table()`, and all callers.
3. **Frontend**: Adds Dutch (`Nederlands`) to `LanguageList`,
`LanguageMap`, `LanguageAbbreviationMap`, `LanguageTranslationMap`,
cross-language form field, and `en.ts` locale.

## Backward Compatibility
- Default language is `"English"`, preserving existing behavior for all
current users
- Languages without a Snowball stemmer mapping fall back to Porter (no
change)
- No new dependencies — NLTK Snowball is already bundled

rag/app/table.py

@@ -548,7 +548,7 @@ def chunk(filename, binary=None, from_page=0, to_page=MAXIMUM_TASK_PAGE_NUMBER,
             else:
                 d.update(stored)
             formatted_text = "\n".join([f"- {field}: {value}" for field, value in text_fields]) if text_fields else ""
-            tokenize(d, formatted_text, eng)
+            tokenize(d, formatted_text, eng, language=lang)
             if _debug_row_idx == 1:
                 logger.debug(f"[TABLE_PARSER_DEBUG] Chunk content_with_weight length: {len(d.get('content_with_weight', '') or '')}")
                 _cd = d.get("chunk_data")
@@ -559,7 +559,7 @@ def chunk(filename, binary=None, from_page=0, to_page=MAXIMUM_TASK_PAGE_NUMBER,
             res.append(d)
     if tbls:
         doc = {"docnm_kwd": filename, "title_tks": rag_tokenizer.tokenize(re.sub(r"\.[a-zA-Z]+$", "", filename))}
-        res.extend(tokenize_table(tbls, doc, is_english))
+        res.extend(tokenize_table(tbls, doc, is_english, language=lang))
     callback(0.35, "")
 
     return res