We could also phrase it more like a sentence, something like

This subkey should expire:
    o when the primary key expires
    o on <Datepicker>

As for showing the maximum validity date, rephrasing to make it clear that this is the maximum date definitely makes sense.

I think we also should invest some time in making KDateComboBox better at handling the range of acceptable dates that's been configured, but that's stuff for a different ticket,

Unfortunately, sentence like UIs are a nightmare for translators. The only thing that works for all languages is self-contained text fragments.

I think the current dialog is mostly okay. We just need to change the text of the first option. "Same validity as the primary key" works for me. I would add the expiration date of the primary key in parenthesis to this text.

Adding the allowed range directly to the second option would make it easier to understand. Maybe "Valid until (between ... and ...):" (iff the primary key has an expiration date).

Then we don't need the extra line with the expiration date of the primary key.