How do I read a single file in a maildir?

Ok, answering my own question here, based on some googling and the helpful comments by mailq.

In short: I installed and used mutt. I had to fiddle a bit with my setup: Inside the directory my_dir where fakemail was creating the mail files, I created the dirs new, cur and tmp and pointed fakemail to my_dir/new. Then I started mutt with

mutt -f my_dir

Now I can review new mails, look at old mails, the umlauts are properly displayed - perfect!

The answer is: Just do it.

Either use APIs in your preferred programming language to parse MIME messages and decode quoted-printable and base64. Or you do it on your own by writing software implementing the linked standards.

Both options work. Pick the right and go for it. (Then look at Stackoverflow for details as this is out of the scope of Serverfault).