Edit after 2 years of my initial review: still no support for many major languages, after 5 years of development it support only some European languages + Persian (in development). The web is more diverse than that, and Firefox has users all across the globe who use more languages.
For instance Mandarin Chinese is spoken by 1.35 billion, Arabic 710 millions (native+second) and Hindi 610 millions (native+second). Yet these three major languages don't even get any interest from Bergamot team, no promises, nothing at all.
I am sad I'm taking away two stars from my initial review, but I still have hope the situation will change, hopefully in this decade.

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I have been waiting project Bergamot since its first news in 2019, and I am excited that it began to fruit in this extension.
I have just tried it with a few pages, and the translation looks very natural, that's a great success.
However, as many has pointed, the languages supported for the moment are very limited. I hope more languages are going to get supported later.