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

web/src/components/cross-language-form-field.tsx

@@ -22,6 +22,7 @@ const Languages = [
   'Vietnamese',
   'Arabic',
   'Turkish',
+  'Dutch',
 ];
 
 export const crossLanguageOptions = Languages.map((x) => ({

web/src/constants/common.ts

@@ -58,6 +58,7 @@ export const LanguageList = [
   'Bulgarian',
   'Arabic',
   'Turkish',
+  'Dutch',
 ];
 export const LanguageMap = {
   English: 'English',
@@ -76,6 +77,7 @@ export const LanguageMap = {
   Bulgarian: 'Български',
   Arabic: 'العربية',
   Turkish: 'Türkçe',
+  Dutch: 'Nederlands',
 };
 
 export enum LanguageAbbreviation {
@@ -95,6 +97,7 @@ export enum LanguageAbbreviation {
   Ar = 'ar',
   Tr = 'tr',
   Ko = 'ko',
+  Nl = 'nl',
 }
 
 export const LanguageAbbreviationMap = {
@@ -114,6 +117,7 @@ export const LanguageAbbreviationMap = {
   [LanguageAbbreviation.Ar]: 'العربية',
   [LanguageAbbreviation.Tr]: 'Türkçe',
   [LanguageAbbreviation.Ko]: '한국어',
+  [LanguageAbbreviation.Nl]: 'Nederlands',
 };
 
 export const LanguageTranslationMap = {
@@ -143,6 +147,7 @@ export const LanguageTranslationMap = {
   Bulgarian: 'bg',
   Arabic: 'ar',
   Turkish: 'tr',
+  Dutch: 'nl',
 };
 
 export enum FileMimeType {

web/src/locales/en.ts

@@ -40,6 +40,7 @@ export default {
       bulgarian: 'Bulgarian',
       arabic: 'Arabic',
       turkish: 'Turkish',
+      dutch: 'Dutch',
       language: 'Language',
       languageMessage: 'Please input your language!',
       languagePlaceholder: 'select your language',
@@ -3064,6 +3065,7 @@ Important structured information may include: names, dates, locations, events, k
       bulgarian: 'Bulgarian',
       arabic: 'Arabic',
       turkish: 'Turkish',
+      dutch: 'Dutch',
     },
     pagination: {
       total: 'Total {{total}}',