First of all - I really have tried searching the site, including FAQ...

I am trying to get a new XOOPS site runnning - v 2.0.9.2. No problem. But then I try to use my old swedish language files for 2.0.7.x. It did not work very well but the site at least still was fully functional.

So I have put a lot of hours translating the english language files for 2.0.9.2 into swedish. But now all I get is blank pages - all over, including admin. I have heared of this language files problem before but I can not find an answer how to solve it. Do anyone know about it?

you have errors in your language-files (opened string constant or missing ; and the end of string).
Possible solutions:
1) Remove folders with your new translations and check texts with goodeditor, which know and can highlight PHP-syntax - you'll find 100% not closed string
1a) if you haven't such editor (bad...) - enable PHP-debug after removing wrong lang-files, return files back ad fix all string with "Fatal Error"
2) Replace your translation with old (2.0.7.3) files, download and install "Xoops Language Checker" from PEAK XOOPS, add all missing constants from english files into yours, using this tool,carefully translate these new strings (they will be added to top of each lang file and commented.
This trick save me a lot of time and attention
3. Install Lang-Tool module and translate all english texts online