I've installed aspell and aspell-en and aspell-si. Which AFAIK is what KDE uses.
And I read about sonnet which supposedly is a KDE spell-checker but it's not doing anything...
For any language...
AFAIK sonnet should be able to reckognize language by it self. But no matter which language I use spelling mistakes go unnnoticed.
I mostly need auto-spell-checker for Konqueror, and so far nothing works.
So which spell-checker is the one KDE uses?
(Windows version uses aspell)
Where can I custumize it, and how can I help to make it better, besides developing it.
But if it will come to that I'll even learn to program in Qt just to get this working .

Sorry for a bit of rant, but it's one small thing which doesn't work.
(Sometimes I want to go back to Firefox )

Have you selected an appropriate language from Konqueror > Settings > Configure Spell Checking? Sonnet does not currently support automatic language detection I believe.

I have but it was set to Slovenian which is I guess "empty". I've changed it to English and it works, silly me.
Any way that I could help with Slovenian dictionary?

I believe the dictionaries are provided by the "aspell-*" packages, therefore you probably need to contact the ASpell project.