[sylpheed:36958] Reply header with german date? / Answer format dialog

Hello,

how can I switch the language of the date output as used in the reply
header to german?

In the options answer format dialog I can change the reply header.
The original looks like this:

----
On %d
%f wrote:

Which results in:

On Wed, 2 Sep 2020 10:45:23 +0200
[email protected] wrote:
----

I changed the words to german:

----
Am %d
schrieb %f:

Which results in:

Am Wed, 2 Sep 2020 10:45:23 +0200
schrieb [email protected]:
----

But the date is still displayed in english "Wed" instead of "Mit".

Is there a way to change this to german, too?


All locales are set to "de_DE.utf8" and sylpheed runs fine with a german
user interface, except for a few missing translations.
Sylpheed Version ist 3.7.0 on Debian Linux.

Best regards,
Christoph