I believe that the spell checking function is provided by your browser, rather than this forum. For reasons that remain a mystery, Pale Moon, the browser I mostly use, reverts to US English spell checking when it updates. I've also had Safari (on a Macbook) do the same once.
The fix is to go to the settings/preferences page for your browser and check that it's set to spell check as you type and that it has the correct dictionary installed. Varies a bit from one browser to another as to how this is set up.