Updated:
Including the other font formatting options on this ticket that was originally just for Monospace:

• Subscript
• Superscript
• Monospace
• Clear Formatting

As there are shortcuts for italic, bold, underline and strike through formatting I would highly appreciate a shortcut for monospace.

By the way: You should use a consistent translation for "monospace". So use either "nichtproportionale Schriften" or "Festbreitenschriftart" as a German translation!