I had this a while back. Apparently if you viewed the site on Android mobile (note that that means Google account across multiple devices), and you just so HAPPEN to use a Japanese keyboard while browsing, it’ll change you default language on the forums to Japanese and won’t turn them back to English on ANY other device browsing with your Google Account.
Solution: Navigate to the Unreal site ON MOBILE, change the language there, and it should properly change it back on all other devices.