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"));
    }

    public void test_detectLanguage_blank() {
        assertNull(languageHelper.detectLanguage(null));
        assertNull(languageHelper.detectLanguage(""));
        assertNull(languageHelper.detectLanguage("   "));
    }