I don't understand how this is related to ProtonMail. It reads more to me like "never use Chrome [if you care about all of your web content being sent to Google]". Sure, ProtonMail is likely to have personal, private stuff in it but so does a bunch of other things, eg. internet banking.
It's related because that translate feature was disabled for those languages, but Google Chrome decided to disregard that specifically on ProtonMail and send the whole thing to Google servers anyways.
It just says they had to turn off the suggest translations feature, which would apply to all sites/languages.