Board Thread:Questions and Answers/@comment-24.118.148.12-20140820044720/@comment-6697950-20140824073435

Written at the beginning of volume 6 to be exact. And the only translations that use "ballet" are old PDFs which were made before volume 6.

Oh, and "ballet" is actually written as バレエ (Baree) in Japanese, according to jisho.