OK I have solved this problem.
The error with the language editor comes with these combinations of settings while using the Url Master module:
1. Lower case Urls switched on
2. Lower case redirect for Urls switched on
3. 'Friendly' Urls for Admin/Host pages.
The problem is that the popup Url for the languages page converts the /locale/en-US to /locale/en-us, which breaks the page.
I think previously this was /language/en-US and has now changed to locale - I could be wrong with that one.
The solution is to either:
a) switch off the friendly urls for the Admin/Host pages
b) change the 'forceLowerCaseRegex' regex pattern to exclude the admin/host pages (as well as anything with a /ctl/ in it, which I recommend). A suitable value is this:
/ctl/|/locale/|/Admin/|/Host/
Save the changes, and restart the site (to clear out cached values) and it shoudl start working OK. Let me know if you find any problems with this solution.