The Linux backend looked like it didn't handle anything non-ASCII at all, but this all needs to be tested.
Windows will be a bit more work because it requires using the Unicode API for creating the main window.
src/posix/cocoa/i_input.mm:95:22: error: use of undeclared identifier 'E_CheckUiProcessors'
src/posix/cocoa/i_input.mm:200:21: error: use of undeclared identifier 'E_CheckRequireMouse'
src/posix/sdl/i_input.cpp:183:19: error: use of undeclared identifier 'E_CheckUiProcessors'
src/events.cpp:540:1: error: pasting formed '::RenderFrame', an invalid preprocessing token
src/events.cpp:541:1: error: pasting formed '::WorldLightning', an invalid preprocessing token
src/events.cpp:542:1: error: pasting formed '::WorldTick', an invalid preprocessing token
src/events.cpp:543:1: error: pasting formed '::UiTick', an invalid preprocessing token
src/events.cpp:544:1: error: pasting formed '::PostUiTick', an invalid preprocessing token
src/posix/cocoa/i_video.mm:559:2: error: use of undeclared identifier 'atterm'
src/posix/sdl/hardware.cpp:85:28: error: ‘atterm’ was not declared in this scope
src/posix/sdl/i_input.cpp:261:10: error: ‘gamestate’ was not declared in this scope
src/posix/sdl/i_input.cpp:261:23: error: ‘GS_LEVEL’ was not declared in this scope
src/posix/sdl/i_input.cpp:263:48: error: ‘GS_INTERMISSION’ was not declared in this scope
src/posix/sdl/i_input.cpp:263:80: error: ‘GS_FINALE’ was not declared in this scope
src/posix/sdl/i_system.cpp:115:25: error: ‘atterm’ was not declared in this scope
src/posix/sdl/i_system.cpp:372:30: error: invalid use of incomplete type ‘const struct dirent’
src/posix/sdl/i_system.cpp:375:42: error: ‘findstate_t’ has not been declared
src/posix/sdl/i_system.cpp:391:12: error: request for member ‘current’ in ‘* fileinfo’, which is of non-class type ‘int’
src/posix/sdl/i_system.cpp:392:12: error: request for member ‘count’ in ‘* fileinfo’, which is of non-class type ‘int’
src/posix/sdl/i_system.cpp:392:56: error: request for member ‘namelist’ in ‘* fileinfo’, which is of non-class type ‘int’
src/posix/sdl/i_system.cpp:393:22: error: ‘alphasort’ was not declared in this scope
src/posix/sdl/i_system.cpp:393:31: error: ‘scandir’ was not declared in this scope
src/posix/sdl/i_system.cpp:394:16: error: request for member ‘count’ in ‘* fileinfo’, which is of non-class type ‘int’
src/posix/sdl/i_system.cpp:401:31: error: ‘findstate_t’ has not been declared
src/posix/sdl/i_system.cpp:403:2: error: ‘findstate_t’ was not declared in this scope
src/posix/sdl/i_system.cpp:403:15: error: ‘state’ was not declared in this scope
src/posix/sdl/i_system.cpp:403:37: error: expected primary-expression before ‘)’ token
src/posix/sdl/i_system.cpp:404:33: error: request for member ‘count’ in ‘* fileinfo’, which is of non-class type ‘int’
src/posix/sdl/i_system.cpp:406:39: error: request for member ‘count’ in ‘* fileinfo’, which is of non-class type ‘int’
src/posix/sdl/i_system.cpp:413:2: error: ‘findstate_t’ was not declared in this scope
src/posix/sdl/i_system.cpp:413:15: error: ‘state’ was not declared in this scope
src/posix/sdl/i_system.cpp:413:37: error: expected primary-expression before ‘)’ token
src/posix/sdl/i_system.cpp:425:16: error: ‘findstate_t’ was not declared in this scope
src/posix/sdl/i_system.cpp:425:29: error: expected primary-expression before ‘const’
src/posix/sdl/i_system.cpp:426:1: error: expected ‘,’ or ‘;’ before ‘{’ token
src/posix/sdl/i_main.cpp:160:6: error: ‘gamestate’ was not declared in this scope
src/posix/sdl/i_main.cpp:160:19: error: ‘GS_LEVEL’ was not declared in this scope
src/posix/sdl/i_main.cpp:160:44: error: ‘GS_TITLELEVEL’ was not declared in this scope
src/posix/sdl/i_main.cpp:217:18: error: ‘atterm’ was not declared in this scope
src/posix/sdl/i_main.cpp:242:11: error: ‘I_Quit’ was not declared in this scope
src/gameconfigfile.cpp:126:27: error: ‘SHARE_DIR’ was not declared in this scope
src/scripting/decorate/thingdef_parse.cpp:929:2: error: use of undeclared identifier 'strlwr'; did you mean 'strlen'?
src/scripting/decorate/thingdef_parse.cpp:937:3: error: use of undeclared identifier 'strlwr'; did you mean 'strlen'?
src/scripting/decorate/thingdef_states.cpp:564:2: error: use of undeclared identifier 'strlwr'; did you mean 'strlen'?
src/scripting/vm/jit_runtime.cpp:957:2: error: ‘unique_ptr’ is not a member of ‘std’
src/scripting/vm/jit_runtime.cpp:957:38: error: expected primary-expression before ‘>’ token
src/scripting/vm/jit_runtime.cpp:957:40: error: ‘nativeSymbols’ was not declared in this scope
src/utility/cmdlib.cpp:962:2: error: ‘DIR’ was not declared in this scope
src/utility/cmdlib.cpp:962:7: error: ‘directory’ was not declared in this scope
src/utility/cmdlib.cpp:962:34: error: ‘opendir’ was not declared in this scope
src/utility/cmdlib.cpp:967:33: error: ‘readdir’ was not declared in this scope
src/utility/cmdlib.cpp:969:10: error: invalid use of incomplete type ‘struct ScanDirectory(TArray<FFileList>&, const char*)::dirent’
src/utility/cmdlib.cpp:973:34: error: invalid use of incomplete type ‘struct ScanDirectory(TArray<FFileList>&, const char*)::dirent’
src/utility/cmdlib.cpp:985:20: error: ‘closedir’ was not declared in this scope
