Set only one language as an installation locale

The `site_languages` parameter can receive multiple language codes separated by
commas. Thus its value cannot be reused as-is as a value of `locale` parameter
which is passed to Drush during site installation.

The value of `site_languages` must be split, and the first element in the list
of languages will be used as a site locale during installation.

drush.py

@@ -129,7 +129,7 @@ def site_install():
         h.fab_run(role, 'chmod ug+w ./sites/default'.format(env.docker_site_root))
 
         if env.site_languages:
-            locale = '--locale="{}"'.format(env.site_languages)
+            locale = '--locale="{}"'.format(env.site_languages.split(',')[0])
         else:
             locale = ''