I don't think the Google Assistant can alternate between multiple languages in the middle of a conversation. So far I've only seen one example of a language learning app, see:
https://tech.busuu.com/creating-a-new-busuu-experience-for-the-google-assistant-on-google-home-e62ba200e000
But they seem to have the same problem as you, being restricted by the limited language options of the Assistant. In the Busuu assistant app they have only one language you can learn, which language that is depends on your locale.
The Google Assistant is a conversation between you and Google. You can
now prompt the Google Assistant in French (“Parler à busuu ”) or
German (“Sprich mit busuu”) and learn English as if you were having a
real conversation with a language teacher. You can already prompt in
English (“Ok Google, talk to busuu”) to learn Spanish.
As for the problem of outputting speech in one language while the Assistant locale is set to another language they use SSML to play an audio sample containing the word/sentence in the "foreign" language.
So it looks like a 2-way language learning app might not be possible with the current state of the Assistant but I hope this will help you.
Also take look at: https://dialogflow.com/docs/multi-language.
