Installing/switching to other language

Hello,

I've recently created a PunBB Forum (1.3.4) and want now to change the language. I've download the language pack for Norwegian, uploaded it into the /lang directory, where it exists now juxtaposed to English.
But when I go into my Administraion settings in the forum, the only lanuage I can choose is still English. Is there any step to take before it works ? Or do I have to re-install the whole forum in Norwegian now ?

Thx for any help.