Of late I've tried out the Translate You Android app. Not necessarily an endorsement of the app itself, but it offers a selection of 9 translator services, a few of which are libre.
Certainly LibreTranslate and ~~Lingva~~ [my bad, it's a GTranslate front-end] Apertium are open source, probably a few more on their list. Give them a try and see which works best for you!
Buggers, removing features that are actually handy to the users ๐ถ Apologies for the snark, didn't know they'd done this.