languageHelper.updateDocument(doc);

        assertEquals("ja", doc.get("lang"));
        assertEquals("タイトル", doc.get("title_ja"));
        assertEquals(123, doc.get("content_ja"));
        assertNull(doc.get("url_ja")); // url is not in langFields
    }

    @Test
    public void test_updateDocument_withNullLangFields() {
        Map<String, Object> doc = new HashMap<>();
        doc.put("lang", "ja");