Is there a similarity-function that minds national charsets?

Hi,

Is there a similarity-function that minds national charsets?

Over here we've got some special cases that screw up the results on
similarity().

Our characters: ä, ö, ü, ß
could as well be written as: ae, oe, ue, ss

e.g.

select similarity ( 'Müller', 'Mueller' )
results to: 0.363636

In normal cases everything below 0.5 would be to far apart to be
considered a match.

As it is, I had to transfer the contents of the table into a temporary
table where I translate every ambigous char to it's 2 char representation.

Is there a solution so that detour is not necessary?