Fallback to cs
While it can make a sense to set it in generic MediaWiki installation (although sk as a fallback to cs is quite unusual(*) - I can't recall any software (except for that where sk is the original development language) doing that, everywhere there is a fallback to en or any other worldwide language), it must obtain a consensus on both sides on relevant Wikimedia wikis.
Also mind phabricator:T13267.
(*) besides fallback from language with higher number of speakers to language with lower number of speakers is quite illogical
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. :)