I have downloaded additional Slovenian dictionary package for TeXstudio and when I run spellchecking, I thought it worked fine, since it pointed out some misspelled words and came up with possible replacements.
The problem is that spellckecker missed some obviously misspelled words.
I made a test and added a gibberish word
asdasdasd in the middle of the document. Then I clicked
Tools > Check Spelling..., but I got a response
No more misspelled words, even marking all the words with
CTRL + a did not help. Nothing did (like placing mouse cursor in the beginning of the document etc.), unless I marked only the word
asdasdasd (by moving mouse over it). In that case, spell checker recognised the misspelled word.
I also copied an article from English Wikipedia, then added the gibberish word in it (and changed language settings in
Options > General) and checking worked fine (but that doesn't prove anything, since the Slovene checker aslo noticed some misspeled words).