Thanks. So I upgraded to 5.1.0 It worked out well. The Slovak language is still there. But in the admin i can see now there are new language strings that need to be translated.
But when I download the .xml file the new strings (not translated ones) are not in that file. That means, that the site adds the new strings to downloaded file only after the strings are translated. Well, OK.
The other problem is that when I write a post in Slovak language with punctuation like for example this:
http://sample.com/forum/všeobecn?-f?...zba-na-str?nku
and when i copy that link form the address bar and then paste it here it looks like this:
http://sample.com/forum/v%C5%A1eobec...a-str%C3%A1nku
Is it possible to fix that somehow? The SEO should be able to remove the punctuation from the the link. I see other vbulletin forums and there the punctuation is removed.