Exactly. Czech and Slovak are (mostly) mutually intelligible.
Shouldn't we set them as MediaWiki fallback language for each other, then?
For pywikibot it has been already set as a fallback language, therefore it could be for MW too, what do you think @Mormegil, Matěj Suchánek, and Danny B.:?
I opened Thread:Portal talk:Sk/Fallback to cs