Ensure the description specifies "Multi-Lenguaje" or "Voces y Textos en Español" so you don't end up with an English-only version.
Currently, there is no official "free" version available for permanent download. Here is how you can still access the game safely: 1. Official Purchase & Remastered Version
, you can right-click the game in your library, select "Properties," and choose the "Language" tab to download the Spanish language pack. System Requirements & Performance Assassin's Creed® III Remastered on Steam