Fantastic language-learning tool - LingoDeer

LingoDeer is a fantastic language-learning tool with a strong Asian-language focus (though it includes a few major European languages, too). 

The app's courses are multilingual, so if you wish to learn Japanese as a German speaker, this is the app for you. 

That said, LingoDeer is also quite challenging, as it features specific, high-level terminology and detailed grammatical explanations. Nonetheless, it is a superb resource, and the developer is constantly improving the app based on community feedback.