Hi Alex,
I'm using standard Joomla 2.5 SEF URLs and Falang for the translation.
The strange thing is that the symptom went away when I did the following:
1. I had changed the New User Redirection page to Home, as I said, in order to work around the problem.
2. I changed it back to same page.
3. I deactivated Joomla SEF (First option)
4. I reactivated Joomla SEF (First option)
The problem had been resolved at step 1, with the work around. I did not check if it came back at step 2. There was no problem at step 3, or 4. Now I have the configuration that had the problem, but without the problem. I assume it was an update problem from one version to the other and somehow flipping switches reset itself.