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 ❤️

[stephanbogner]

@mhilbrunner and @AThousandShips Thanks for your help ❤️ Should be good now (hopefully)

[stephanbogner]

Ok damn. Will have to get the build tools working locally to be more efficient 😅

[stephanbogner]

@mhilbrunner @AThousandShips

Seems to work now:

Local build output

``` make FILELIST='tutorials/i18n/internationalizing_games.rst ' html sphinx-build -M html -d _build/doctrees '.' "_build"/html tutorials/i18n/internationalizing_games.rst Sphinx v4.4.0 in Verwendung Build language: en, i18n tag: False Lade Übersetzungen [en]…erledigt loading pickled environment... erledigt building [mo]: targets for 0 po files that are specified building [html]: 1 source files given on command line updating environment: 0 added, 0 changed, 0 removed looking for now-outdated files... none found preparing documents... WARNING: search index couldn't be loaded, but not all documents will be built: the index will be incomplete. erledigt writing output... [100%] tutorials/i18n/internationalizing_games generating indices... genindex erledigt writing additional pages... search erledigt copying images... [100%] tutorials/i18n/img/localized_name.webp copying downloadable files... [100%] tutorials/scripting/gdextension/files/cpp_example/SConstruct copying static files... erledigt copying extra files... erledigt dumping search index in English (code: en)... erledigt dumping object inventory... erledigt build abgeschlossen, 1 warning. The HTML pages are in _build/html. ```