I worked on a language learning app for about a year. I wanted to read the first Harry Potter in french and easily create flashcards. I iterated early on based mostly on what I wanted to see in the app instead of talking to people. I definitely should have talked to people much more and much earlier. Also I didn't have any other founders with product or design experience. In retrospect I would have really benefited from other cofounders.

In addition to this such an app is very content driven and the lack of being able to use anything other than public domain content really limited me. It's still up in a semi working state https://unchart.io. Personally it achieved my language learning goal as I was able to read most of the first Harry Potter in french so I will probably continue to add more features as a personal project.