The numbers in this dialog come from system font setting for monospace fonts and that might be broken for you. But you should then have problems in other applications, too. There is nothing special here and it works for all our other users.

Can you check if something messed with your system fonts
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes]
[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes]

And I think after the language change you did not fully restart Kleopatra. You have to use "Datei -> Beenden" to close it fully, otherwise it only gets minimized in the tray and the language does not change.

I am leaving this on needs triage for now since I don't really know what could cause this and would first like to see if something is in your FontSubstitutes.

Thank you for the fast response!

The section [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes] looks pretty normal

The section [HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes] does not exist.

When peeking the content of the control using Accessibility Insights from Microsoft (https://accessibilityinsights.io/) I get the following tooltip window when hovering a combo box list element within the dialog.

And the inspection window of Accessibilty Insights also reports a very strange content of the Name property of the list object

As Accessibility Insights very likely uses the same font for displaying all property values, it seems really to be a problem with the content of the list element rather than its visual representation.

But you were right regarding the problems with the language change. I was not aware the Kleopatra only minimizes itself to the tray when the close button is clicked. After fully closing and restarting the application, the language was changed correctly. Unfortunately the problem described above is independent of the language selected.

To me this looks more like a ki18n/Qt issue than a font issue. In particular, the key size drop down doesn't use a monospace font. The code uses the default locale to localize the number representation. What's the system language of your Windows?

The system language is German, the entire system is a German PC, German keyboard layout etc. Other languages used are English and Spanish.
The system is heavily used with different applications including SW development tools, etc.
Never noticed issues like this, so I am pretty sure it's something special in Kleopatra...

I think it's something special in Kleopatra in combination with your system. Kleopatra is deployed on loads of computers in Germany and you are the first one to report this problem. I understand that you do software development. Did you maybe set some localization settings or environment variables to test/debug things you develop? Can you try some other KDE application, e.g. Kate? You can get it from the Microsoft Store or alternatively at https://binary-factory.kde.org/job/Kate_Release_win64/.

No, I didn't make any special localization settings or environment variables on my computer. The only multi-lingual use case I have is that I used for some time the spanish version of Microsoft Office.

The same problem occurs also in Kate

and in Okular

Hard to believe that I am the only unicorn in the universe having this problem. Nevertheless I would understand if you don't want to investigate any further. But I would appreciate your help and contribute as much as I can.