NOTE:

the default language is set, defined and "burned-in" during your tenant setup.

You CANNOT change the default language.

This impacts all users in your tenant.
They will ALLWAYS receive notification in the language defined during SETUP.

Meaning:
If you have set FRENCH during setup and change to e.g. English or whatever else. System Messages like SharePoint Storage limit is reached will be in the setup default language. In this case here French!

The Organizational Information setting below will not change the default language to users or admins. !