File name completion on Mac OS X with German umlauts

[Markus]

Hi,

I'm using a German Mac OS X 10.5.2. I wanted to open a file 
called "rückerstattung.txt". So I typed C-x C-f rü <tab> ... but 
it didn't work. I thought this was a bug in Aquamacs, but then I 
tried to do the same with that console emacs which is bundled 
with Mac OS X 10.5. Same issue.

Then, I remembered that some time ago, I had the same issue 
within Terminal where typing "rü<tab>" didn't autocomplete the 
file name.

So, can't the autocomplete within Emacs work because it doesn't 
work under Terminal?

I assume there must be an Emacs guru around who is running a 
German Mac who can help me out with this annoying behaviour.

Thanks in advance,
Markus