In Bitcoin Core GUI I don’t see the option to choose Slovenian (sl) as the interface language. I had it selected in v 0.21.
Slovenian is over 98% translated (only some obscure messages are still left untranslated on Transifex – I myself did a large part of the translation) and I think it should be included.
I find it strange, however, that Slovenian seems to be included in the source code: https://github.com/bitcoin-core/gui/search?q=sl
(Also please note that Slovenian / slovenščina / sl is a distinct language from Slovakian / slovenčina / sk.)