Multisite Language Dialects

I have WP Multisite installed with six different languages, which are all showing up in Settings, in the drop down (Hebrew, English, Croatian, Czech, Hindi, etc)

The problem is, I need to also have Mexico spanish and Peruvian spanish. I dropped in the es_PE.mo (peru spanish) in wp-content/languages but it is not showing up in the Settings language drop down, nor any other kind of language dialects.

In fact, ‘es’ is registered as “Spanish Castilian” under wp-admin/includes/ms.php

Is this a bug, or what am I doing wrong?
Thanks in advance!