This is a must.

Please make it possible to translate everything to your client's language. Automatic based on language selection (with the ability to overwrite individual strings to fit your brand voice).

Or alternatively as a start let users translate each text string manually, in case you don't have ressources to do certain language in-house.

The customer experience suffers greatly when exposed to half-translated emails and fully english booking flows, for a non-english speaking business that has everything else in their native language.

Not everyone speaks your supported languages and for the rest of us this is a problem.