Studied #Esperanto 20 years ago and Dualingo confirmed that I have forgotten almost nothing. I'm not a big fan of EO, but I think this language should be in schools, simply because it's a great base for learning other languages.
Discussion
Doesn't seem like it would help with something like Russian, maybe Spanish or European languages though, I could see that.
There was a language called Slovio. Literally Esperanto with Eastern European languages.