Input encoding in groff macro files

The -K option allows me to specify my troff file's encoding, but this does not seem to extend to macro files. If a custom macro file contains non-ASCII characters in a string, the output looks like this:

Mit freundlichen GrüÃen,

My setup looks like this:

# letter_to_doe.tr
.recipient John Doe
.text Sehr geehrter Herr Doe ...

# letter.tmac
.de recipient
.\" ...
..
.de text
.\" ...
..

Mit freundlichen Grüßen

How can I properly convert the non-ASCII characters in the tmac file?