macOS app: get system language from QLocale

Fixes: #288

syncplay/messages.py

@@ -46,9 +46,15 @@ def getMissingStrings():
 
 
 def getInitialLanguage():
-    import locale
     try:
-        initialLanguage = locale.getdefaultlocale()[0].split("_")[0]
+        import sys
+        frozen = getattr(sys, 'frozen', '')
+        if frozen in 'macosx_app':
+            from PySide2.QtCore import QLocale
+            initialLanguage = QLocale.system().uiLanguages()[0].split('-')[0]
+        else:
+            import locale
+            initialLanguage = locale.getdefaultlocale()[0].split("_")[0]
         if initialLanguage not in messages:
             initialLanguage = constants.FALLBACK_INITIAL_LANGUAGE
     except: