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. :)