For flashcards/spaced repetition I've used Anki quite a bit.

https://github.com/ankitects/anki

I did get good enough at Chinese for a 3-week trip but I only needed to order food and navigate streets.

I have no idea how good it is for "fringe" languages.

As a language professor, I can tell you that 2-hours a week will only maintain your level of a language. Ideally, setting aside time every day is the most important thing.

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.

I have neither used a dedicated app specifically for learning the language, nor completed "learning" the language to a passable level, but I did start learning japanese using only freely available online resources. There was quite a lot of information in video and text form regarding grammar etc.. For training vocabulary and the japanese characters (I only learnt Hiragana and Katakana - no Kanji) I used Anki. In my oppinion it's very doable to learn a language this way without a course (though keep in mind that your pronunciation will suck if you don't get feedback from people that actually speak the language) - in the end I was just too lazy to learn vocab.