Spanish translation

Hi

This is the first time I create a pull request.

I've translated the en.php file to es.php using Transifex at https://www.transifex.com/projects/p/elgg-plugins/language/es/

I'd like you add this translation to the main repo. Next days I've been working on other plugin translations (spanish and galician too). I'd like to add my translations to the main repos of each plugin.

Thanks
dani