I tinkered around a bit more, and if I enter a parameter in Checkmate it causes an error that reveals the string that is sent to the LT API:

So I can't enter any parameters because Checkmate adds to the base URL before starting the parameter string. My first question is - why does it say "language=en" instead of "language=en-GB"?
Here is a screenshot from my session settings:
More in the line of feature requests, if there was a way to add parameters, even with a text field, that would be cool.
This might all be my bad. I might be missing something. Thanks for your help.
Daniel