Fallback to cs

1. Yes, it's possible. We have for instance a reciprocal fallback between pt and pt-br.
2. Coverage is ok for the most frequent extensions, but not for all of them. Maybe this change is going to affect/help mostly non-Wikimedia users. Is English less surprising/annoying, for the average user? (I'm not aware of data other than language proficiency statistics, to answer this question; maybe we can think of something else too?)

Special:LanguageStats/cs vs. Special:LanguageStats/sk can give an idea: MediaWiki extensions are respectively 54 % and 27 % translated, so we have at least 27 % messages in sk which would use the fallback. I'll try to make a list to see how much overlap there is.

@Mormegil: It is better than nothing. Czech users are still more like to see Slovak translation if Czech translation is missing and vice versa than to see English/German/Polish strings or nothing at all.