Re: Own mail address in heading replaced by "Ik"

> Apparently nobody else seeing this (or similar in their own language)?

No…

>> My wife and I both use Kmail 18.12.3 on openSUSE 15.1.

That version of kmail is quite old… So it may be that this is functionality 
that has since been removed.

>> My wife complains that, wherever her e-mail address should be in mail
>> headers, this is replaced by the string "Ik" (which is Dutch for "I").

I remember vaguely that this was a feature of some header themes. Have you 
tried to change the header theme? (View > Headers)

You can in principle make your own header theme, but documentation is 
sparse.