I was hoping this could be easy, but it's not. I need to add the Czech spelling dictionary to Mathematica.
I copied the two files: cs_CZ.aff and cs_CZ.dic in to the /Applications/Mathematica.app/Contents/SystemFiles/Components/SpellingData
I opened the file "SpellingData.m" and added there the "Czech" -> "cs_CZ" like it's done with other languages.
I went into the Option inspector and Global Options -> System configuration -> Dictionaries and added there the necessary associations.
However, I still cannot make it work, i.e. it still underlines everything red when I write in Czech.
Do you have any idea how to make it work?