SAK-26268 - Mexican Spanish translation not working properly in Sakai 10 (Merge into 10.x)

Added "_es" locale copying "_es_ES".

git-svn-id: https://source.sakaiproject.org/svn/gradebook/branches/sakai-10.x@309906 66ffb92e-73f9-0310-93c1-f5514f145a0a