Comment 10 for bug 219931

It seems that you are not using ja_JP as your default locale. Therefor, the system is switching between Chinese and Japanese fonts while typing. This is because Pango doesn't know whether you are typing Chinese or Japanese, as the characters (even Hiragana) are included in both font sets, but rendered a bit differently in regard to punctuation marks. This is actually a feature of Pango and not a bug.

To tell the system, that you prefer Japanese fonts in any case you'll need to type the following in a terminal window:

fontconfig-voodoo -f -s ja_JP

(when selecting Japanese as your default session language in language selector, this would be done automatically for you).