How to improve on a language that you've already covered here on Duolingo
Imo, read Wikipedia articles in that language about people you know, so you can develop more ranges of vocab (hey, I grew a lot from reading Sri Lankan pages on the Dutch Wikipedia)
oh and use other sources too
I like reading newspapers online and also reading books on the Kindle app, which has built in dictionaries. Both help with reading ability. For speaking it doesn't and I'll probably need to find some people to chat with.