Hola Ramon,

1. What language you need?
2. I tested on 2 sites, and I was able, so probably there is different reason, not missing lang file.

Spanish (es-ES) // No Mexico yet

Download & Install in Joomla Admin "Extensions: Install": es-ES.com_easystore.zip

Notice! That translation was done a few months ago, so it not include all new phrases. In that case, you have to compare English version of files from those folders:

• administrator/language/en-GB
• language/en-GB

and add new phrases and translate them, especially from 2nd folder