If the strings are hardcoded to asni, it'll be extremely difficult to convert them all to wide strings, which is what utf-8 supports.
If that's the case, then what you'll want to do instead is swap the character set/ code pages to the correct ones, and if you plan to distribute a multilingual version of the game, then the best course of action would be to use a proxy dll to edit the code base on conditions either by user options or by system settings. you might want to look into createfont.
Hope that helps, if you need a service you can let me know the offer and I'll see if it's worth doing the work. Preferably if you can communicate in Fluent English , that will be best