I think "the microphones are listening when they're off" is still a conspiracy theory at this point. It's not really needed to get enough information.
Are there any ways that Google could find out that you're interacting together?
- Do you share an IP address/router?
- Do you watch YouTube videos in that language?
- Do you use any messaging apps where you speak that language with other people, but also speak with your partner?
- Do you access any Google services (with a Keyboard for that language installed) that your partner also accesses?
- Do you use location services that could pinpoint both of your locations to the same street address?
- Does your partner interact with any of the people you're learning that language with? (E.g., Social media friends, "Contact"s, live in the same geographic region)
- Is your device on the same network as your partner's*? (Wi-Fi or Bluetooth)
I'm not saying these are all ways that Google uses, but I believe that each of them are ways that Google would be able to associate that language to your partner.