Anybody having problems comparing two different language versions of a PageProvider page?

Background:
We have a customer with 18 different languages on their site running CMS 6 R2. They use a custom page provider for serving some of the pages.

Problem:
When the customer compares two different language versions of the same page (served by the custom PageProvider) they actually end up seeing two pages of the same language. When they select e.g. English and Swedish as the languages to compare, they see two english pages.

When I compare the IFrame src URLs they are as follows:
English version (left panel/iframe): /en/Products/Some/Product/?id=12435__PP&idkeep=True&epslanguage=en
Swedish version (right panel/iframe): /en/Products/Some/Product/?id=12435__PP&idkeep=True&epslanguage=sv

In both these pages EPiServer disregards the epslanguage query parameter and loads the english language version (apparently since the URL starts with "/en/". In the case of ordinary EPiServer pages (not provided by a page provider) this works but when the pages are served by a page provider, the URL language parsing does not work.

Anybody else having this problem? Is this a known issue/bug?
How can it be fixed?
As I see it, the only way that we can fix it ourselves is if we write a custom URL rewriter with might introduce new "features" (bugs).

I thank beforehand for any assistance anyone can provide.