Spanish translation

Spanish translation from HerculesWS/FluxCP
Translation improvements

Changed some translations to be neutral and friendly between spanish
variations like: es_ar es_cl es_es es_pe es_co etc
User sidebar lang support
Added two missing labels

Re-added two missing labels in es_es.php and en_us.php
Ref. 202b7ea
Minor fix

Fixed a variable name to make the Register link works with lang support.
Merge pull request #20 from Freyja-Folkvangr/spanish-translation

Spanish translation