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OK,
After restoring the test database locally and seeing the issue and also a couple of niggly bits like making sure localhost:xxxx was one of the host names etc...
I could click on that globe in the header while in edit mode to view the pages outside the CMS. But noticed in Admin mode it was still trying to view the pages on another host.
Finally chanced changing the main site URL to match the localhost:xxxx and all worked as expected, even with the language switching in Edit mode.
So the URL you use to access the CMS should match the main Site URL in the website settings.
In the CMS editor when usind the "Sites" tab on the left hand panel to change the site language it reloads the homepage instead of staying on the current page/block.
This only appears to be happening on our test server at the moment. When I do the same thing running the site locally with local database it works as expected.
If I point my local version to the same test database then I get the same problem which leads me to believe its something stored in the database causing the issue but I have no idea what it might be.
Anybody got any ideas ?
I've looked at the Admin | Config settings but can't see anything that stands out as the potential cause.