There are a couple of language learning tools I know of that are FOSS, the first is of course anki, which you know of, the other two are both focused around reading, specifically Lingq like tools:
- LWT (Learn with texts), it's a self-hosted PHP/mysql piece of software, the GUI is very... basic. If you can see past it's GUI and are fine figuring out how to set it up, it does work pretty well, but it just lacks a lot of polish so requires patience.
- FLTR (Foreign Language Text Reader), similar to the above except slightly easier setup. It's a java program. The feature set is quite basic though and I found the overall experience lacking, even compared to LWT, but it might be something to try.
If you're not familiar with how Lingq (and the above two lingq-inspired tools), you provide it content and then it highlights unknown words, you can click the words which show you what they mean and you can then add it to a set of learning words. Over time through these words coming up naturally, you learn them. I've found it just pretty much lets you more easily read interesting texts earlier, because looking up words is far easier.
Hard to answer but maybe Haiku or GNU Emacs