https://www.google.com/accounts/NewServiceAccount?service=transconsole&continue=http%3A%2F%2Fwww.google.com%2Ftransconsole&followup=http%3A%2F%2Fwww.google.com%2Ftransconsole

as spotted here:
http://www.google.com/robots.txt

This service is pretty old.

Indeed, this service is old. However, I've just realised that you can use that link to add / remove languages instead of emailing translation-servicesgoogle.com like the Google In Your Language page recommends.

I can't seem to add/remove languages though that page, and emailing Google hasn't got a response 1 year on.

D'oh! I just found it.