With a literal /č/g you'd have to worry about the file encoding (the file containing that code). If you look up the individual characters you want to replace in Unicode they'll be called "U+ABCD SOME TEXT DESCRIPTION" (where ABCD is a hex number). You can copy that number into a regex like
/[\uABCD]/g
Given that č is U+010D, ć is U+0107, š is U+0161, đ is U+0111, and ž is U+0173,
/[\u010D\u0107\u0161\u0111\u0173]/g
will match those five characters. As for exactly what you want to do with this information, I'm not sure either.
Side note: I'm pretty sure there's no "Croatian" block in Unicode, otherwise this would be a lot easier. Also Javascript doesn't support the \p{} construct you can find in Perl and PHP/PCRE, so you can't specify complete alphabets (like how a \w represents [a-zA-Z0-9_]).