When editing a child-language (a language with the parentlanguage set) in admin/lang.php you only see (1)the definition, (2)the English definition and (3)the currently selected language's definition. If, however you are editing a childlanguage, a language with a parentlanguage, a field might already exist in the parentlanguage and I believe it would be best to show this in the editor instead of the English definition (or in addition to it). This would make the language editor show the actual strings used in Moodle.

Currently when extending a language, you see a lot of missing fields, even if they are defined in the parentlanguage (admin/lang.php?mode=missing) and this can cause problems.