To get Mail.app to correctly show the 24 hour time format, do this:
- Quit Mail.app if running
- Navigate to Applications in the Finder
- Right-click or control-click onto Mail.app and choose "Show package contents"
- Open Contents -> Resources -> German.lproj
- Find the file named "Localizable.strings" and make a backup copy in case anything goes wrong.
- Open the file in a text editor and find the line that reads "12HOUR_TIME_FORMAT" = "%1I:%M"; and replace it with "12HOUR_TIME_FORMAT" = "%H:%M";
- Save the file.

