For classical releases, one language per release is suggested (Style / Classical / Track Title - MusicBrainz). For the others, the suggestion is to generally yes, enter both, however the tracklist does it: often “One Language (Another Language)”. I see your release has two completely separate tracklists though, which seems a bit different - was I entering this I would probably just enter it in English as from the back cover, but I understand that you might want to also give weight to the Bulgarian titles. Maybe still do “One (Other)”?
Hopefully we’ll finally get the alternative tracklists option this year and then we can do this properly.