1. Open with a text editor program like Notepad the following file: "ACShadows.ini"

Located in:
"C:\Users\*Username*\Documents\Assassin's Creed Shadows"

2. Find this line:

Sound=en-US

3. Modify this line with your desired language. For example:

en-US (English)
es-ES (Spanish)
de-DE (German)
...etc

Example result:

Sound=es-ES

And that's all! Enjoy the game with your voice language.

REMEMBER: Unfortunately, this must be done every time we run the game. If we close the game, the .ini file its restarted, OR YOU CAN TRY THE FOLLOWING:

I've been testing it, and if you're not going to change anything in the settings, a solution to keep the file always in your language is to go to ACShadows.Ini > 'Properties' > 'Read-only'. That way, the game won't be able to modify the settings, and they will remain changed.

Update 03/23: 4 days later, this change keep working without problems with the Read-Only activated.