Hi, and thank you for your interest!
Tails is being translated using Weblate, a web tool designed to ease translating for both developers and translators.
Translation project for Tails into Spanish currently contains 6771 strings for translation and is 68.4% complete.
If you would like to contribute to translation of Tails, you need to register on this server.
Once you have activated your account just proceed to the translation section.