Actually the user name field can handle multi-byte character (glyphs? code points?) perfectly fine (assuming you have an appropriate font) the problem is that the game uses an ancient ANSI windows function to retrieve the user name, which returns it encoded with whatever code page windows has configured while the game expects UTF8
Yes the battlescape issue is most likely related: ancient ANSI windows function to get the home directory path, which is again returned with whatever encoding windows has configured, the game then chokes on it while trying to load the battlescape: the game and SDL — which we are using to load the game library — expect the path with UTF8 encoding
I've been working on replacing the old ANSI functions for widechar equivalents — and handling the necessary UTF8 <==> UTF16 (which is BTW windows native encoding: ANSI functions are provided for compatibility reasons) conversions — on master, if someone who uses windows and has an user name (and user directory) with non ASCII characters could test that would be welcome