@Harri-K, user interface language is defined by your base language in the course (the language you are learning from). It's assumed you are pretty fluent in that language, as you are using it to learn another one. So, you switch languages by switching the course. At the moment, you can't remove a course from your account, just ignore it. Lingvist people have plenty of free space on their servers, so it's not a problem at all.