- set locale to US English UTF-8 for POSIX targets

Locale-dependent standard library functions didn't treat UTF-8 strings correctly, e.g. iswalpha() returns 0 for any non-latin letter
The same function from MSVC runtime classifies such characters as alphabetic even with C locale

https://forum.zdoom.org/viewtopic.php?t=65641&start=18#p1115930

src/posix/cocoa/i_main.mm

@@ -199,8 +199,8 @@ int OriginalMain(int argc, char** argv)
 	// Set LC_NUMERIC environment variable in case some library decides to
 	// clear the setlocale call at least this will be correct.
 	// Note that the LANG environment variable is overridden by LC_*
-	setenv("LC_NUMERIC", "C", 1);
-	setlocale(LC_ALL, "C");
+	setenv("LC_NUMERIC", "en_US.UTF-8", 1);
+	setlocale(LC_ALL, "en_US.UTF-8");
 
 	// Set reasonable default values for video settings
 

src/posix/sdl/i_main.cpp

@@ -170,9 +170,9 @@ int main (int argc, char **argv)
 	// Set LC_NUMERIC environment variable in case some library decides to
 	// clear the setlocale call at least this will be correct.
 	// Note that the LANG environment variable is overridden by LC_*
-	setenv ("LC_NUMERIC", "C", 1);
+	setenv ("LC_NUMERIC", "en_US.UTF-8", 1);
 
-	setlocale (LC_ALL, "C");
+	setlocale (LC_ALL, "en_US.UTF-8");
 
 	if (SDL_Init (0) < 0)
 	{