To change the language settings on Ubuntu 18.10 (18.04) open the user's by click on the the upper right corner and click on the settings button. Then choice Region & Language from the left side of the Settings application.
By click on the + button (1), you can add a new Input source. According to the question, probably you need Bulgarian (traditional phonetic). When there is more than one Input source the language switcher will appear in the toolbar (2). You need to log-off and log-in (or reboot the system) to apply the new Input source.
By the button Options (3) you can change the behavior of the language switcher when you switch the windows. Also there is an instructions how to change the keyboard shortcut, which by default is Super + Space.
If you click on the button Manage Installed Languages (4), initially you will be asked to install an additional common language support tools and then you will be able to add extended system support for Bulgarian. For example the Bulgarian spell check in LibreOffice will be available.
By the option Languages (5) you can switch the system language. In this case I would keep the original names names of the user's folders. By option Formats (6) you can change the Date/Time/etc. display format.