If you just swap the 8-bit encoding to UTF-8 then you can get subtle bugs, for example truncating strings is not so straightforward anymore. Windows programs can also natively support Unicode, however converting an existing one to Unicode is hard if you only use char not TCHAR types. You'd still need to decode the input files from UTF-8 encoding to UTF-16.
The other problem is the fonts themselves. Clausewitz uses bitmap fonts. Although the font definition files could theoretically specify codepoints higher than 255, the amount of additional characters is still limited by the size of the bitmap files. So don't expect there to be a full set of Latin script charaters included. Not to mention other charater sets like Cyrillic of Greek.