When setting "Bulgarian (български)" as the language for a cPanel account, all the text in cPanel still shows as English.
Bulgarian is currently not a supported language and should not be listed as an available language.
We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-39798. Follow this article to receive an email notification when a solution is published in the product.
There is currently no workaround available at this time.