Zkuste toto:
SELECT a.locale, c.text AS text_1, d.text AS text_2
FROM (SELECT DISTINCT locale FROM i18n) a
CROSS JOIN table_1 b
LEFT JOIN i18n c ON b.text_1 = c.id_i18n AND a.locale = c.locale
LEFT JOIN i18n d ON b.text_2 = d.id_i18n AND a.locale = d.locale
Ukázka SQLFiddle
UPRAVIT :Tohle by mohlo fungovat lépe:
SELECT
a.locale, b.text_1, c.text_2
FROM
(SELECT DISTINCT locale FROM i18n) a
LEFT JOIN
(
SELECT b.locale, b.text AS text_1
FROM table_1 a
JOIN i18n b ON a.text_1 = b.id_i18n
) b ON a.locale = b.locale
LEFT JOIN
(
SELECT b.locale, b.text AS text_2
FROM table_1 a
JOIN i18n b ON a.text_2 = b.id_i18n
) c ON a.locale = c.locale