I find Slavic languages to be less distant from one another than other major Indo-European languages. They're by no means mutually intelligible when spoken, but you can kind of get by. There's also the Interslavic [1] conlang, which is targets mutual intelligibility without much additional learning. It's surprisingly readable, but like most conlangs, I think more of a solution is search of a problem than anything.
[1] http://steen.free.fr/interslavic/