The API documentation for webkit_web_context_set_spell_checking_languages() only mentions that it affects the Accept-Language header in requests, but says nothing about navigator.language(s) and the Intl default locale, which are also affected by it because in the end using the function results in a call to WTF::overrideUserPreferredLanguages().
One could also argue that the function should not receive a WebKitWebContext because it operates on a global setting, but let's just document all the implications for now and leave that for another bug ;-)
Created attachment 453824 [details] Patch
Committed r290825 (248061@main): <https://commits.webkit.org/248061@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 453824 [details].
<rdar://problem/89806783>