I think if you add (nl.lng and nl.t9) write language make true but the phone language not true.because you mut flash fs what support nl for phone and it make true phone language.
Othe way for example:
Second: when you have the file language. Rename it to one of the file language that is in your fone that you don't use.
Up load that new file language to phone.
Go to general / Language and choose the language you just add. Enjoy.
Exampe: how to did with my fone k800-->k810i
I am france , so i need FS-m east n africa (English - france - ...)But the newest firmware of k800i R8BA024 doesn't have M east n africa yet. They do have FS APAC(Malay - Philippine-Tagalog - Vietnamese - Chinese )
so i use FS APAC and want the france language.
i know i wont use fr language so i rename the vietnamese language file(vi.lng) to france language file(fr.lng) and upload the new language file to phone.
TPA\Preset\System\Language .
it worked perfect.