I think it would be better to use the local translation of the device, in iOS you can now select and request to translate with downloaded languages.

Maybe use a button that activates this option if the user requires it, or also leave it on automatic. 

 Glad to hear iOS has offline translation support. But there's web clients and Android clients too. 

 I think firefox added something like that. Wouldn’t be surprised if there was an official translate api soon 

 Android has Google ML Kit for offline translations.

https://developers.google.com/ml-kit/language/translation/android