What I want to achieve is even if the user is on japanese version of my Grave site mysite.com/ja/when he click a link in navigation that leads to forum, it will take him to mysite.com/forum/ not mysite.com/ja/forum.
I think you want to look at the include_default_lang setting.
@rhukster but whether I have this enabled to false or true, if user is on language other than English, in my case - Japanese (/ja/) which is not a primary language, but one of secondary languages I use on site, clicking at link will take him to mysite.com/ja/[urlpath] rather than mysite.com/[urlpath].
That’s how Grav knows what language you want.