I don't understand: what language are you trying to export to? My take
was that you export the doc to latex/pdf/html and you tangle the code
fragments to some arbitrary file (a file that can be fed to a prolog
interpeter to be executed).

> This is a (lisp)programmers task. The usual proceeding would be to look
> at the list of extisting ob-<language>.el files and pick a language that
> is very similar to the new one (in case of prolog maybe a difficult
> task?). Then try to adapt this file to your new language -> ob-prolog.el.

Assuming that the statement I quoted is true, one should be able to
export files that contain prolog code fragments:

#+BEGIN_SRC prolog
foo
#+END_SRC

One has to do `:exports code' since exporting results would require
evaluation which *would* require that one write an ob-prolog.el.

Also, one should be able to tangle the prolog code fragment(s) into
files (no ob-prolog.el needed).

Exporting and tangling  are generic services that babel provides even if
there is no evaluation for that language.

But as I said, I have not tried it: I am only trying to interpret what
the doc is saying, so when the eating comes, the pudding may be stale.
If the interpretation is wrong, I would love to be corrected. If the
interpretation is right, but the exporting/tangling does not work, then
that would be a babel bug.