Set default_locale: where to find the reference and what for?

I am hesitating in setting that in system/config / system.yaml but I don't see the point / utility.
What form should it take for France.

I am may be stupid being a beginner but I tried nearly all the expressions and it still do not work...

whatever you set in your user/config/system.yaml (never use system/config/system.yaml btw), is passed directly to a setLocale() method within Grav:

https://github.com/getgrav/grav/blob/develop/system/src/Grav/Common/Grav.php#L212-L217

So, you just need to set an appropriate value that PHP understands:

http://php.net/manual/en/function.setlocale.php

default_locale: fr

Full list can be found here: http://www.loc.gov/standards/iso639-2/php/code_list.php

I set it simply with default_locale: fr and it works!
Thank you
I tried before with default_locale: (LC_ALL, '') and it works also (but I don't really understand what it means.