I have now tried installing Tiki 1.8.2 and 1.7.7 on my workstation, to see if I can reproduce the problem in 1.7. The answer is, I can't!
This therefore looks like a bug introduced in 1.8
It would be very nice if it could be corrected (apart from the fact that I imagine it would be a real show-stopper for all the many non-English users of Tiki), since I would love to be able to use the "switch language" modules which seem to have been introduced with 1.8.
All the better to see it corrected since in my experience, Tiki is one of the best CMS for language support.