Janko,
Here is a proof that Unicode is definitively possible in Squeak. I put on a same web page some English, French, Slovenian, Russian and Chinese:
http://mivsek.eranova.si:8888/
This is done by help of Aida/Web on Squeak 3.9. This image has Unicode patch installed but it would probably work the same on plan 3.9 too.
This could be a hunch and could be completely wrong, but have you tried to *use* Slovenian in Squeak, in the sense that type in from keyboard, display in workspace, and use it in class names/variable names, etc.?
To try it, take the OLPC image for example, gunzip the attached latin2.out, and evaluate the following in a workspace:
StrikeFontSet installExternalFontFileName6: 'latin2.out' encoding: Latin2Environment leadingChar encodingName: #Latin2 textStyleName: #DefaultMultiStyle.
And then, evaluate:
Locale currentPlatform: (Locale localeID: (LocaleID isoString: 'sl')).
and save the image. If you are on Windows, this would let you type latin2 characters, display them, and use it everywhere in the image...