[ix101]

Are there any apps specifically targetted at polyglots? It's a niche so I doubt it. I guess you can tell the app that you're native french speaker learning German if you are more proficient in French and picking up German. This way you are training two languages simultaneously.

[nickjantz]

What do you mean by targeted at polyglots? Almost all apps allow you to chose your base and target language. Laddering like this is very common.