Formal (as in "formal event"):
* svečani (pronounced: sveh-cha-ni) - This is the most common and general word for "formal".
Formal (as in "formal language"):
* službeni (pronounced: sloo-zhbeh-ni) - This word means "official" or "formal" in a more bureaucratic sense.
Formal (as in "formal attire"):
* svečana odjeća (pronounced: sveh-cha-na od-yeh-cha) - This phrase literally translates to "formal clothing".
You can also use formalni (pronounced: for-mal-ni), which is a direct translation of "formal", but it's less common than the other words.
The best word to use will depend on the specific situation and the context.