To translate the voices you need to buy the original Half-Life game from Steam and install it to get the files.

The folder you need is: valve_spanish

Once you have it, open the following path:

C:\Program Files (x86)\Steam\steamapps\common\Half-Life MMod\valve

Copy and REPLACE the content inside the valve_spanish folder to the previous path.

Now you will be able to enjoy the game with the voices in Spanish.

Enjoy :)