Gathering all my divine powers I deduct you're on Linux and Godot is set to 'no' but' no' is not installed and you get the error in a terminal when you start Godot.
Now there would be a workaround with the standard locale C but you probably want 'no' (edit: if that exists, may also be a typo in some configuration field or file).
If it is a Debian based installation you can check your locales with 'locales -a'.
To install a locale run 'dpkg-reconfigure locales' as root or with sudo and choose from the list.