How to add a language in forums?

In the module forum i see only en.php & ru.php

How to add a language?

Adding nl.php is not enough... i don't see it.

place the nl.php file in
boonex/forum/install/langs

Then goto admin => modules=> forum...
when the admin forum starts you see compile language EN, RU and NL

click Compile llanguage NL.

Done!!

I see only: 

There are a lot of forum posts about this. Maybe search a bit more....
Another thing you can try is do to admin modules manager (Toevoegen modules) and check forum and scroll down and recompile. dutch (Compileer talen)