restrict editing standard language files and support automagic creation of local language files

Since Moodle interface allow editing language files, user tend to modify standard files to customize the language for local sites. However, it has long been recommended to use either local or child language to do such customizations.

It may be a good idea if the language editing interface by default created _local version of the language files with the strings edited by users. Editing standard files should still be possible, since it is needed for, for example, translators, but should require extra hoops to go through to get into that mode.

Furthermore, the admin interface could automate producing child languages. After all, it is not that different from editing local language except for having to create the language config.