I'm guessing that it's the default for the forum software that we use (XenForo).
Alternatively, I always assumed that the spell checker was built into the specific web browser you were using?? I.e. mine is Chrome. Which is still in American, however....
EDIT: just checked in Google Language settings, and it is indeed set to English (American), which the spell check uses. It is possible to change it to actual English, however it says this language can't be used for spell checking.... #facepalm