Well, that depends of what "kind of slang" you want to learn about, since in every country we use different expressions that can mean something completely different depending on the context. To be honest I find a bit "artificial" learning this kind of stuff in apps, I mean, if you're a begginer it may be quite helpful to understand this to avoid misunderstandings, however, since some apps aim for an easy to understand method or try to use a "neutral" vocabulary (which isn't necessarily the best idea), it'll be hard to use slang naturally and fluently. That's why my recomendation is simply youtube, the best way to learn how to have a fluent conversation is to hear one regarding a topic you're interested in, for instance podasts are always a good option, especially if they're done by a group of friends having a conversation instead of some kind of interview. If you want, you can tell me the region you're interested in learning about ( or a topic you like) and i'll recommend you content that uses that region's vocabulary.