I want to make a site that supports two languages. Am I allowed to have 1 IPB for each language, besides the test one?

You can just install an additional language pack and let users choose which language to use.

but then I'd have both languages mixed. I want people with different languages to have sepperate areas.

In that case, you'd need two separate licenses.

I'll have to handle two languages with one installation, then.

You could probably do it with a mod and a bit of tweaking and have two sections visible for guests etc.

IPSBeyond would be the proper place to ask for help / advise if needed with this :)