I'm writing this from Arch, but in this aspect I think Mozilla Firefox works just the same. Do you mean that if choosing to install a language from this link,
https://addons.mozilla.org/en-US/firefox/browse/type:3, Firefox doesn't install it?
I've never encountered any issues doing that. When writing you just right-click with the mouse to open up a menu which among other things has a language selector; simply tick the box of the preferred language.
I prefer these extensions, but if it for some odd reason doesn't work you could install Google's tool bar.
Its for spellchecks. I want firefox to be in swedish and openoffice to be in swedish

As alomst every dist offers Firefox language files in package manager, but Pardus dont...not yet anyway.
The dictionaries, spellchecks works fine to get in swedish...But thats not it.