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:
0_1549912404403_Spell check_English (Canada) dimmed.png