Anki is absolutely awesome. Since there is a large amount of community made decks, I found one that was specifically made for the book I used for learning Japanese. In addition to that, I also found a free Japanese-English dictionary app that had the option to add words as flashcards to a specified Anki deck. Even though it doesn’t have the most modern looking or intuitive UI, Anki is my main tool for learning Japanese because there are so many useful integrations and a large community behind it, and it is completely free.
Maybe not exactly what you are looking for, but you could use a website archiving solution to store one of those online man-page collections. I know it sounds like overkill, but it’s actually easy to setup and can be used for all kinds of other things as well. For instance, I have a local docker container running SOSSE for C++ and Python references and for all kind of libraries and APIs I use for software development. This way I have everything I need on my laptop and can work anywhere, even if there is no network connection.