I moved your thread to this part of the forum, seems to me to be a better place.
The sources files are free, you do not have to pay to
translate Pegasus Mail (and you're not paid to do it). To translate PM
to another language you have to be a team with a team manager, some
testers and you need David Harris approval, then you have access to the source files.
I think that there is
already a Dutch translation team so you won't have DH approval for a
new team, contact them and ask them to be part of the team.