I am unable to access in Japanese.
This site japanese.stackexchange.com statistically functions in English, but it is perfectly fine to ask and answer questions in Japanese too. You may come across some questions/answers that have both English and Japanese versions.
Some discussions on meta related to this:
Users can post in any language, but the site is essentially in English.
I believe StackExchange is owned by an American company, so the site is mostly in English.
It seems that they have 2 forums in foreign languages, one in Japanese and another in Russian, but both seem to be for computer programming: