i18n: use gettext.bindtextdomain() instead of locale's one

It doesn't work on OSX (per #67) and even so, the documentation says it
should not be used unless in specific cases, which we are not
considering in Sonata.

For more information, see
https://docs.python.org/3/library/locale.html#access-to-message-catalogs

sonata/launcher.py

@@ -30,7 +30,6 @@
     sys.exit(1)
 
 import gettext
-import locale
 import logging
 import os
 import platform
@@ -103,7 +102,7 @@ def run():
 
     gettext.install('sonata', locales_path, names=["ngettext"])
     gettext.textdomain('sonata')
-    locale.bindtextdomain('sonata', locales_path)
+    gettext.bindtextdomain('sonata', locales_path)
 
 
     ## Check initial dependencies: