Internationalization and pay.nl does not seem to work properly together, any idea how to fix that one? I am working on a website with four languages.

We use Ubercart with external payment facilities. We have Paypal and the following specific to Netherlands: eBanking, iDEAL, MisterCash. That works fine.

Now we are trying to get pay.nl to work. This one offers more methods of payments.

The request uses /nl/ as expected with a path that looks like this:

http://www.example.com/nl/order/paynl/notify/?...snip...


But the i18n module seems to change that to /en/ instead:

http://www.example.com/en/order/paynl/notify/?...snip...


And that prevents the payment from being accepted.

Any idea what could be happening? Would there be a way to prevent translations from happening when hitting /order?
Your problem might be because of the "Language negotiation" configuration. 

So check this configuration, which in D6 you can find via /admin/settings/language/configure/language. Experiment with the 4 available options in the Language negotiation selection list, to see if any of these options solve your issue.