It should be enough to add Japanese as a preferred language in the browser settings. At least this was sufficient when I tested it last year.
By the way, my IME2Furigana userscript, that you installed yesterday, automatically adds the lang="ja-JP" attribute, so that it always uses a Japanese font: 直す.
Also, maybe this thread should be moved into another category – “Japanese Only” is intended for threads where we only write in Japanese. But don’t ask me which one is the correct category 
Probably just Japanese Language => Questions?