I did not mean to suggest that editing should be done by teachers. You read more into my words that I wrote. My suggestions had exactly the site administrators in mind. I agree that editing languages requires reading the documentation. But let's face it, Moodle has lots of administrators that are not technically trained. This is one of its advantages actually. And it is the reality that many people read the instructions only when they run into trouble.
The issue is, Moodle's interface, as it is now, encourages to edit standard files, which is in opposition to recommendations.
I disagree that language editing is done mostly by translators. As far as I can see, there are quite a few Moodle sites which modify (customize) a number of strings and there are some that do quite extensive changes. Check forums for how often people are advised to not modify standard language files but create own language or the local files for small customization. We have some 80 languages, so with let's say 3 translators per language, it is barely over 200 people for whom the interface is designed. I bet that far many more Moodle sites have customized languages (some may have only a few strings).
We have locally modified quite a few language files (in more than one language), including having our own language. Since I have to maintain them through Moodle upgrades, I tend to watch posts related to language editing on moodle.org. I feel that since the switch to utf8 and changing the location of language files, there have been a visible increase of issues related to dealing with those files. Utf8 in particular created issues for direct editing. Also more and more sites seem to be run on commercial ISP's and direct editing requires there extra steps.
I do not see roles as the solution here. Roles can be used to control who specifically is allowed to edit the langauges, but that is a different issue. I am after having proper, or at least better, support for language editing through the web interface by administrators not only by translators. A simple solution could be to have separate links for editing standard language files and for customizing language files. I think such a distinction would make it clear what's going on and it should not be that difficult to have an internal file location pointer that tells the code whether we are editing master file or local extension.