The spellcheck of Czech text is not working correctly if you select Winsoft dictionary in all versions of CC2020: it marks all words starting with lower case letter as wrong and recommends capitalising ALL words. Unfortunately the Winsoft dictionary is the only option for Czech language as both Hunspell and Proximity are really bad.
1 Type Czech text
2 Set Winsoft dictionaries in InDesign preferences
3 Run spell checker
4 All words in lowercase will be marked as misspelled (each word is treated as if it stands at the beginning of a sentence)

Expected result: see CS6, Winsoft dictionaries work well in this version.