Using Interpreter Mode couldn’t be easier, all you have to do is ask the Google Assistant to turn it on and away you go.

By using the mic on your phone to listen to nearby speech, Interpreter Mode will transcribe foreign language in real time, before spitting out a written translation in your language of choice
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From there, you can continue letting the Google Assistant do its thing, or say something in your own language and watch as Interpreter Mode goes the other way and translates that into the other language, allowing for real-time back and forth conversation.