Try this:
UPDATE t1
INNER JOIN (SELECT t2.id, GROUP_CONCAT(t4.language ORDER BY t4.language) languages
FROM t2
INNER JOIN t3 ON t2.id = t3.X_id
INNER JOIN t4 ON t3.languages_iso2 = t4.iso2
GROUP BY t2.id
) AS t2 ON t1.id = t2.id
SET t1.languages = t2.languages;