I made a program in Excel 2010 VBA that contains letters like ć č š...
Msgbox("Čiča gliša")
works.
Looks like Excel 2013 supports those letters in cells and formulas, but not in VBA.
VBA replaces them with some symbols which aren't even on the keyboard.
I get errors executing the code.
I believe it's something to do with language settings.
As BambiLongGone stated, weird string are not likely to work. I would say your best shot is looking at this article. There 's a tool called Unicode to VBA that you can use to convert all your string in your application. For example :
Čiča gliša
will be converted to
ChrW$(&H10C) & "i" & ChrW$(&H10D) & "a gli" & ChrW$(&H161) & "a"