We cannot allow user to overwrite it. So the only way to change it is here, and it changed it for all applications linked to this SALT installation.
If we allow overwrite in each application, and if two applications use different value, classes generated for SALT framework will became unconsistent :
the I18N_DEFAULT_LOCALE set the parent class of all generated classes, so we cannot have a class A with parent B for an application, and class B with parent A for another.