I'm surprised that Firefox Translate includes Bulgarian but not Russian in its list of available languages. It seems to me that the latter language is far more important.
I understand the translation quality is so terrible because the model size is 30 MB? It is difficult to understand the meaning of the translation. It would be cool if you made it possible to choose the size of the model , for example - 30 MB - 300 MB - 1 GB
I've translated some Wikipedia articles, and it's ok for a machine translation, it's comprehensible at least. Grammatical case and gender of words is often inconsistent, in my experience larger size model should fix that.