assertEquals("ja", doc.get("lang"));
        assertNull(doc.get("title_ja"));
        assertEquals("コンテンツ", doc.get("content_ja"));
    }

    public void test_updateDocument_withoutLangFields() {
        languageHelper.langFields = new String[] { "description" }; // Different field

        Map<String, Object> doc = new HashMap<>();
        doc.put("lang", "ja");