Use “gettext” instead of “locale” for L10N

As the documentation on the “locale” module states, the “gettext” module
should be used instead. Therefore adjust the localization calls to use
gettext instead.

Additionally fix one button label which did not use localization at all
and update the message catalogues.

mcg/application.py

@@ -3,7 +3,7 @@
 
 import gi
 gi.require_version('Gtk', '3.0')
-import locale
+import gettext
 import logging
 import urllib
 
@@ -123,7 +123,8 @@ class Application(Gtk.Application):
 
     def _setup_locale(self):
         relpath = Environment.get_locale()
-        locale.bindtextdomain(Application.DOMAIN, relpath)
+        gettext.bindtextdomain(Application.DOMAIN, relpath)
+        gettext.textdomain(Application.DOMAIN)
 
 
     def _load_ui(self):