With kmail 5.2.0 (kdepim-6.4.0) my email addresses in the headers and CC lists are changed into "Ik", that is Dutch for "I"
This is very annoying.
How can I change this? I like to see my email address and I have a few, so I can see which one is used.

As far as I understand, this is not (yet?) configurable. You may want to file a report on bugs.kde.org.

kees61, did you already file a bug report?

I neither like the new behavior and want to join your report.

Yes, somebody did: https://bugs.kde.org/show_bug.cgi?id=362280

According to http://commits.kde.org/messagelib/bb8aa ... 63b2e467ac (which will be in the next stable release), "Me" will not be shown when you have multiple identities. Would this fix the issue?

Yes, for me that would fix the issue.
Thanks.

einar wrote: "Me" will not be shown when you have multiple identities. Would this fix the issue?

Can the outgoing/sender address be displayed, even for the single identity case? That would be my intuition for a default, and there would be nothing wrong with setting an option not to display the outgoing address.

For now, it can't. Please follow up on the bug report posted above (but do remember the Code of Conduct when posting, as some commenters did not).