Fallback to cs

Wikimedia wikis don't automatically rule MediaWiki i18n; there are hundreds of MediaWiki wikis in Czech and i18n is there to benefit as many users as possible.

There's nothing illogical about a reciprocal fallback, see also the referenced research; the "bigger" language, as defined by number of translation, will simply use the fallback less. Per the list provided above, cs messages falling back to sk are 7 times less than sk to cs. You can probably translate them all in an afternon. :)

Please read properly what I wrote. Both here and in commit message. That would prevent you reacting with useless post.

In a nutshell - by default yes (I don't care), on WMF wikis obtain consensus.