On enwiki pages, the ContentTranslation extension causes a prompt to appear in the languages sidebar, above any other languages present, suggesting the page can be translated to en-gb.
In this case, the "Other projects sidebar" is also present, but I can replicate this using a testing account with no preferences/gadgets other than ContentTranslation enabled.
On clicking the link it launches ContentTranslate to ask for a page title, and then fails when it tries to open the page on en-gb.wikipedia.org.
This may mean either (or both) of:
* it's not checking whether the suggested language exists as a project; and/or
* the list of possible languages isn't sanity-checked to remove unwanted local variants like en-gb
- both of which are probably unintended.
I'm viewing this from the UK and I am guessing the prompt is either due to browser localisation or to IP geolocation, but it's not clear which.