SELECT random_book.*
FROM book
LEFT JOIN (SELECT * FROM book ORDER BY RAND()) random_book ON (book.lang = random_book.lang)
GROUP BY random_book.lang

SELECT * FROM book WHERE lang=1 ORDER BY RAND() LIMIT 1
UNION
SELECT * FROM book WHERE lang=2 ORDER BY RAND() LIMIT 1
UNION
SELECT * FROM book WHERE lang=3 ORDER BY RAND() LIMIT 1