i18n: Add explanation for locales and automatic language setting

[stephanbogner]

As discussed in #9684 I think the docs could be enhanced by adding the following:

• Explanation on locales and language
• A simple example script to set the game’s language based on the player’s OS language

This is my first commit for the docs so I am not sure:

• If the writing style is correct
• If I used the .rst syntax correctly

So help is very much appreciated ❤️