Is there any privacy-preserving app to learn new languages?

Pirate children’s tv shows in that language and watch them
- This is a great suggestion, thanks!
  
  You might pick up some casual phrases but unless you are spending a ton of time with them breaking it down, you’re now an adult, and you will need more than the shows.
  Just become undocumented in another country. 😂

There's Anki which is one of the most popular flashcard apps. Kiwix is pretty great for having tons of offline content from websites like Wikipedia, StackExchange, and Khan Academy, but I've run into a few bugs with it. I believe the current version isn't on F-Droid but they plan to remove the non-free build tools in the next version. There's the translation dictionary QuickDic. There's some language specific apps like Der Die Das and Starke Verben for learning German, Kakugo, Fun with Kanji, and Kanji Dojo for Japanese. There's several language specific dictionaries like Nani?, Nheengaré and a PReVo. For learning numbers/time there's the Nanji clock widget that can show the printed time in several languages.

The absolute best app for learning a new language is from pimsleur.com but their privacy policy is trash.
Second best is italki and their privacy policy is trash.
YouTube can also be helpful but you need to use alternative ways to access it if you want privacy.
If you really want to learn and preserve privacy, you'll probably need to interact with native speakers daily.

- Of course books are very valuable, but I would say that for language learning, at least for me, they are insufficient since they lack people pronouncing the words
  
  There's also text books that could be considered, they usually include listening exercises, though I can't say how good that is as a method for self learning
  
  There are probably some language learning videos on YouTube (wich can be beaten into respecting your privacy with newpipe, revanced or similar things)