You could join to translation tables separately, and pick the first non-NULL
translation with COALESCE
, like this:
SELECT a.*, COALESCE(t1.translation, t2.translation, t3.translation) as translation
FROM aTable a
LEFT JOIN aTableTranslate t1 ON aTable.id = t1.aTable_id AND t1.language like 'en'
LEFT JOIN aTableTranslate t2 ON aTable.id = t2.aTable_id AND t2.language like 'jp'
LEFT JOIN aTableTranslate t3 ON aTable.id = t3.aTable_id AND t3.language like 'cn'