Are you a total novice or do you have some basics? Because Duolingo is a decent introduction. That said, I'm quite the fan of Clozemaster. I've only been using it via the webpage for 20 days now and alongside Duolingo, but I like the approach for what is essentially five minutes a day. Obviously neither are free nor open source. For free and open source, I've been intending to take a look at Ankidroid, but haven't gotten around to it yet.