Ensure cocoa_get_lang() returns a valid locale or None

See #1233

kitty/cocoa_window.m

@@ -14,6 +14,7 @@
 // Needed for _NSGetProgname
 #include <crt_externs.h>
 #include <objc/runtime.h>
+#include <xlocale.h>
 
 #if (MAC_OS_X_VERSION_MAX_ALLOWED < 101200)
 #define NSWindowStyleMaskResizable NSResizableWindowMask
@@ -419,7 +420,13 @@ - (void)openFilesFromPasteboard:(NSPasteboard *)pasteboard type:(int)type {
         locale = [[NSLocale currentLocale] localeIdentifier];
     }
     if (!locale) { Py_RETURN_NONE; }
-    return Py_BuildValue("s", [locale UTF8String]);
+    // Make sure the locale value is valid, that is it can be used
+    // to construct an actual locale
+    const char* locale_utf8 = [locale UTF8String];
+    locale_t test_locale = newlocale(LC_ALL_MASK, locale_utf8, NULL);
+    if (!test_locale) { Py_RETURN_NONE; }
+    freelocale(test_locale);
+    return Py_BuildValue("s", locale_utf8);
 
     } // autoreleasepool
 }