Drupal Multilanguage domain specific with PORT not working
I'm pretty confident that the reason for your issue / problem, is because Language detection by domain only works on port 80. That issue has 'some patches' attached to it', and status = Needs review.

Not sure if you want to try/consider experimenting with any of the patches in this issue though.

Something else you may want to check is your configuration at Configuration -&gt; Languages -&gt; detection and selection: the order specified there could be what explains the current behaviour in your site.

You'd have to experiment a bit with what the exact order in your case should be. In my experience, I often get these kind of issues to work by using an order like 'URL, Default, User', and not something like 'Default, URL, User'. So try to experiment a bit with the various combinations in this order to see if it helps.

Curious to hear if either of those options will somehow help ...