en_?? ?

Problem

When replying to or forwarding a message in Outlook, the attribution header uses a 12-hour clock (AM/PM).

You are of the opinion that this format is a remnant from the Dark Ages, when it was, well, dark and people had to be told whether it was day or night, and so you would really like Outlook to stop that.

Analysis

Outlook is being excessively clever here, as usual. It has an option to override the language used for formatting the attribution header, and this option overrides not just the language but the entire locale with the (im)moral equivalent of en-US.

Solution

In Options/Advanced/International options, turn off “Use English for message headers on replies and forwards and for forward notifications. [sic]".

Outlook will now use the format defined in Windows regional settings. It will also use the language defined there.