#: ../userhelper.c:197
#, c-format
msgid "%s's password"

This doesn't look good in English, and it gets even worse when translating
into Swedish.
Swedish doesn't use an added apostrophe and an s, but just an added s to
imply ownership.
So "menthos's password" would be "menthoss lvsenord" in Swedish.

Could there please be some logic added to detect if the username ends in
an s or z, and then two different messages used?

This applies to the message
"In order to run \"%s\" with %s's privileges, additional information is"
"required."
too, as it has the same type of "%s's" problem.