I can confirm this for Opera 58.0.3135.53 on Windows 7 32-bit (stand alone installation).
If I remove all entries except "English (Canada)" from the list of languages, "English (United Kingdom)" will be re-added after a restart of Opera.
And "English (United Kingdom)" is also the language, which I could activate under "Spell check", whereas "English (Canada)" remains grayed out.
The screenshot shows the situation after restarting Opera:
It's not an adblocker problem.
If you view the source of the page (ctrl + u), you will see this line:
<iframe src="//thepiratebay.org/static/si0Eim0u/exo_na/sky1.html" width="120" height="600" frameborder="0" scrolling="no"></iframe>
This is the line that is causing the problem.