How to translate plurals in language with multiple cases (most of European)
format_plural() does support multiple plural versions in general.

format_plural() only has arguments for singular and plural because english only has those two and the input is always english, Drupal does not support translating interface text from another language than english.

However, it is something that defined per language and not per word, that doesn't work. I'm not aware of a workaround that can handle per-word differences.

Actually, according to https://en.wikipedia.org/wiki/Czech_declension#Plural_forms, those rules (1, 2-4, 5+) are always the same? So that should just work.

This is how it looks in the UI