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.)
<img width="561" alt="Screenshot 2021-09-14 at 14 12 21" src="https://user-images.githubusercontent.com/294075/133255485-7c4ec5b4-e78b-45b4-b672-7e40c35d9cbb.png">