I don’t think there’s a languages menu in the interface settings by default, so for those that use stock or stock-based clients (like the HD client), you would have to replace your data dir (enUS) with your locale’s dir from a source like WotLK 3.3.5 HD project on Discord: https://discord.gg/Gyb9Z252vt
They offer locales such as enES (Castillian Spanish) and enMX (Mexican Spanish)
Just download the locale of your choice (about 2 gb), open your WoW folder, enter the “data” subdirectory, delete or move the existing enUS folder, then drop your new locale into there!