Google also provides an official on-device translation SDK that is free-as-in-beer (https://developers.google.com/ml-kit/language/translation).

The models are around 20M for any single language so there is a not insignificant cost on the user in terms of delay for downloading, data usage and disk space.

Disclaimer: I work for Google and worked on the ML Kit Translate SDK, but I don't speak for Google.