doc.put("lang", "ja");
        doc.put("title", "タイトル");
        doc.put("title_ja", "既存のタイトル"); // Should not be overwritten
        doc.put("content", "コンテンツ");

        languageHelper.updateDocument(doc);

        assertEquals("ja", doc.get("lang"));
        assertEquals("既存のタイトル", doc.get("title_ja")); // Should remain unchanged
        assertEquals("コンテンツ", doc.get("content_ja"));
    }

        "title_gl",
        "title_gu",
        "title_he",
        "title_hi",
        "title_hr",
        "title_hu",
        "title_hy",
        "title_id",
        "title_it",
        "title_ja",
        "title_ko",
        "title_lt",
        "title_lv",
        "title_mk",
        "title_ml",
        "title_nl",
        "title_no",
        "title_pa",
        "title_pl",

        "title_gl",
        "title_gu",
        "title_he",
        "title_hi",
        "title_hr",
        "title_hu",
        "title_hy",
        "title_id",
        "title_it",
        "title_ja",
        "title_ko",
        "title_lt",
        "title_lv",
        "title_mk",
        "title_ml",
        "title_nl",
        "title_no",
        "title_pa",
        "title_pl",

        "title_gl",
        "title_gu",
        "title_he",
        "title_hi",
        "title_hr",
        "title_hu",
        "title_hy",
        "title_id",
        "title_it",
        "title_ja",
        "title_ko",
        "title_lt",
        "title_lv",
        "title_mk",
        "title_ml",
        "title_nl",
        "title_no",
        "title_pa",
        "title_pl",

During crawling and indexing, Fess:

1. Detects language from document content using Apache Tika
2. Validates against `supported.languages` list
3. Creates language-specific fields (e.g., `content_ja`, `title_en`, `content_sv`)
4. Applies language-specific analyzers for better search results

Configured in `fess_config.properties`:
```properties
indexer.language.fields=content,important_content,title
```