Fallback to sk
Can we set Czech as fallback of Slovak and vice versa? If we do this, when a translation is missing in Slovak but available in Czech, the translation in Czech will be shown to Slovak users instead of the English original (and vice versa for Czech).
According to Lindsay, there are formal intelligibility studies that found written Czech and Slovak to be 98 % intelligible in both directions. The source might be Nabelkova, M. 2007. Closely related languages in contact: Czech, Slovak, "Czechoslovak". International Journal of the Sociology of Language 183: 53-73.
On the other hand, according to Eurostat and CLDR, English is known by less than 30 % of the population in CZ and SK.