"... patient's personal information was also compromised," said Chris Hacker, Special Agent in Charge of FBI Atlanta.

What a name.

As far as apps, people sometimes use Anki and get a card pack (commonly the most used words or phrases), but that is more of a supplementary tool.

Duolingo also has a website, if installing the app is an issue. Just use adblock in this case.

What app did you find?

Easer? It's also on F-Droid. It's clunky though, so it takes time to get into it.

Edit: It seems to not have a silent mode either as far as I can tell, but I'm gonna keep looking.