I have a release in South Estonian language (or more specifically in Mulgi) and I would like to have an option to select it when adding a release.

Sometimes Mulgi, Seto & Võro are actually considered their separate languages, but sometimes specified as dialects of South Estonian language. I don't know which one is more correct, I'm not an expert. But I do know they sound fairly different from Estonian.
Creating 3 separate language entries might not be very useful though, since it is a dying language and already very few people speak it, let alone release records, that use it.

More info here: https://en.wikipedia.org/wiki/South_Estonian_language