How to keep the languages I speak alive?
Right now I can fluently speak my native language and English and my German is slowly becoming fluent and I understand French, Dutch and Spanish as long as they're not too complicated. But I learn German, French, Dutch and Spanish all from Duolingo. How do I keep these languages alive in me? My teacher once said that if you never use a language it dies in you. Is the solution to repeat the Duolingo skill trees or what?
Repeating the trees is an option. Listening to media in the languages you spoke of is also an option. Perhaps read a book in those languages as well! There's no substitute for speaking but you can keep comprehension alive and well =).
You could watch movies and listen to music/radio in those languages. Follow groups and individuals on social media. Also books, magazines and websites. Anything that you do in your native language you can do in your target languages.