Cannot remap umlaut keys

[Andreas Voegele <voegelas@gmx.net>]

I used to remap the umlaut keys in CC Mode but the following key
bindings no longer work when Emacs 21.3.1 is run under X.  The
bindings work if Emacs is started on the Linux console though.

(add-hook 'c-mode-common-hook
 '(lambda ()
    (local-set-key [?\366] "{")    ; ö -> {
    (local-set-key [?\344] "}")    ; ä -> }
    (local-set-key [?\326] "[")    ; Ö -> [
    (local-set-key [?\304] "]")    ; Ä -> ]
    (local-set-key [?\337] "\\"))) ; ß -> \

Is there another way to remap the umlaut keys that works under X and
on the Linux console?