How to SELECT from three different tables using multiple foreign keys
It's pretty obvious.  You JOIN to languages filter out the null rows because there's no matching to the languages table.  The combination of RIGHT JOIN and LEFT JOIN is bad, but it should work.

    SELECT m.module, m.content
    FROM `modules` m
    LEFT JOIN contents c ON m.id = c.module_id
    LEFT JOIN languages l ON c.language_id = l.id
    WHERE l.tag = 'en-US'