Hello,
Sorry, my knowledge is not good enough to answer in Spanish and I do not know the translations of the menu entries.
You have to manage this via the Joomla! main menu "extensiones".
- The first entry in the drop down menu should be something like "administrar".
- - There you might find a first entry like "instalar" where you can upload your downloaded language package.
- - There you might find a third entry like "administrar" where you can check the activation (and activate if still needed).
This is not specific to JEM.