Ansi means you only support a single codepage. and can thus you typically only support languages for the single codepage the program was designed for.
You may be able to make your program support multiple codepages, but if you need to go there, it's far easier to go unicode.

The advantage of Unicode is that you have more than 256 different types of characters in your character set.
The disadvantage of unicode is that you have more then 256 characters in your character set. (yes, I'm doing that on purpose).

Or putting it another way, you gain the ability for all the glyphs, but it also means you have to deal with that.
(you'll be using wchar_t instead of char, which is 2 bytes on Windows (and thus actually not unicode but either UCS2 or UTF16), 4 bytes on unix. It means you can't make common assumptions such as if a string has 14 characters (wchar_t) it'll have a display length of 14 symbols. It could be less since you have combing diacritics. you can't just take substrings by length etc etc etc.
Or essentially every single interaction with strings in your program needs to be re-evaluated for correctness.