Hi xmorera,

First thing you have to do is be sure that Spanish is enabled for your users. You may install a Spanish Core language pack or create one and have Spanish enabled as a valid Language for your portal. For this you may "Add a New Language" or "Install a Language Pack" at the bottom of Admin-Languages. (Tip: do not forget to click the "Edit Language" link under the Language dropdown list, with "Español" selected, then check that the Enabled option is marked).

Second thing, once you have Spanish enabled for your portal, you may edit the Text Resource Files, under Admin-Languages: Select Spanish under Available Locales, then from the tree at the left select the proper files and edit the text resources (Local Resources - Desktop Modules - Events). When you save your changes pressing Update at the bottom of the page, the new resource file will be created/updated.

Don't forget to choose the proper "Mode" option: System, Host or Portal (the latter AKA "Admin"). Check this old blog post for reference.

Another thing you may do is load into your site a Spanish Language Pack that already contains the Events module translated to Spanish.

Saludos!