I would give 3 stars, however I got Mate for free as I was early supporter. 4 stars deserved in my case there is no alternative.
Safari plugin is disappointment.
1. It detects wrong language on websites. English websites like gate.tv, coinex.com, Czech websites and many more. I've disabled translate to Turkish and now Mate detects it is french language instead of english. If I remember correctly, this function cannot be disabled. I've disabled safari plugin.
2. Translate by double click sometime works but most of time it doesn't.
It would be nice if Mate was as tuned as google translator in UX design. Investment in better UX would be awesome, however I understand such investment would probably not be profitable.
Wish you many users and awesome updates.