Chosen select's autocomplete does not work for multibyte characters such as Japanese.
When you type in Japanese into a select box chosen does not search for options containing the Japanese characters entered.
This is a firefox specific issue.
Further info on this issue. Firefox handles input by IME, it requires the user to click ENTER to convert what they typed letters to the Japanese/Chinese/Korean characters.
In chosen clicking ENTER selects the first list value and renders the search box useless in Firefox.
The jqueryUI autocomplete project has had to deal with the same issue, see similar bug reports on their project here:
Use http://ivaynberg.github.com/select2/ if you want chosen styling and use multibyte charsets.
#2017 may solve this issue.