Japanese and Chinese use their own whitespace symbols that MySQL does not understand.
Make sure that the words in the texts you are going to index are separated with ASCII separators (spaces, commas etc). Anything outside the ASCII range will probably not work.
Besides, you'll probably need to fix ft_min_word_len: by default, MySQL won't index words shorter than 4 characters, and most Japanese and Chinese words.
In Cyrillic languages transilteration errors are quite common.
All letters from this sequence: АВЕКМНОРСТуХ / ABEKMHOPCTyX are indistinguishable in most fonts.
The worst of them is Cyrillic С/ Latin C: both these symbols are located on one key on the keyboard and do not differ at all in most fonts, but they have different codes.
MySQL will not catch it either.