I’ve studied languages on and off since I was six, yet I’m not fluent in any of them. Honestly it’s kinda embarrassing; and frustrating. As far as I can figure, the main reason I’ve lost so much of what I’ve learned over the years is due to the fact that I don’t have anyone to speak it with. Watching tv and movies in the other language helps, but it's not nearly as beneficial as having a conversation and really exercising those neurons. 

I don't know about you, but I text pretty much everyday. It's not an all-day thing, but I do message either my friends or family regularly. So why not use all those conversations as a way to practice another language? If we leverage Google Translate for both incoming and outgoing messages, you could not only practice reading, but also writing in the target language. Being that android allows you to choose your default messenger, it'd be easy enough to switch to Exchange and wouldn't necessarily require the other person to have the app. 

Instead, Google would provide a sort of "language filter" 

for anyone you text normally. And personally I find this far more manageable and comfortable when you compare it to the available services where you skype with a language coach half way around that world.

One of the challenges would be building trust in the Google Translate algorithm. This could be solved by displaying the native language as you type a new message and/or allowing users to quickly swap back and forth. Empowering people to also help improve the algorithm would not only build trust through a sense of personal investment but would ultimately improve the accuracy of the algorithm in the long run.

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Making the app accessible to people of different experience levels would also be important. The biggest barrier there being a sense of confidence when it comes to typing in another language. This is where I think having 

access to a conversational AI directly in the app 

would allow people to get answers to questions that would otherwise force them back in to their native tongue.

Improving the accuracy of translations would be the main point of growth. I'm curious if this is where users could elect to have a handful of messages from a less private conversation vetted by a native speaker. And if this wasn't built by Google, I see it becoming a part of a language learning course from Babbel, Duolingo, or Berlitz.