Cloud inferrance is not private: These providers, even if they don't get hacked, have bribed employees, or outright lie, cannot and will not resist a subpoena or similar.
Mozilla is acting unethically by obfuscating what's on offer here and overstating the privacy and security properties that can be achieved.
It's doubly unfortunate because simple translations are completely reasonable to do with cpu inference.
Late thought: consider what telegram went through over just end to end encryption...
And practically every country has lower privacy standards for data that crosses borders.