How to avoid insert anything without switching to read only

[Lennart Borgman]

In viper-mode nothing should be inserted if you are in vi mode. It works 
for all keys that are assigned commands (like i, o, a etc), but it does 
not work for national characters. Those are not assigned to anything in 
viper. I suggested to the maintainer that this should be fixed by 
advising `self-insert'. Those national characters are bound to 
self-insert according to help (C-h c). Michael was very helpful and 
added this.

However to my surprise it does not work. I have tried to find out why. 
It seems like the adise is not called. I suspect that it has something 
to do with key-translation-map where it looks like those characters are 
translated with `encoded-kbd-self-insert-ccl' if I understand this 
correctly.

How can this be handled?