airtime_mvc/application/models/Locale.php
inside of the directory you downloaded LibreTime - to contribute a PR you need to use git. If this isn’t something you are comfortable with I can help with that part if you share your translated .po
At some point a .mo file needs to be generated from the .po file. I’m not sure at what step in the process this happens, perhaps when you are running the install or it may require an additional command. Basically the .mo file is a compiled binary from the human readable .po file
You can see this commit for an example of recent modifications for the Russian translation - https://github.com/LibreTime/libretime/pull/985/commits/5f3310da03a1642b3d45e80f218e50be4b672e70
Sorry this is so complicated but we haven’t built the proper infrastructure yet to make translations easy and I appreciate your interest in helping add an additional language.