We try to set this using testRunner.setPOSIXLocale("en_US.iso88591") but this doesn't seem to be working on Big Sur for me locally (navigator.language is still returning "en-gb" in the test). Presumably something here has gotten broken for non-US locales due to lack of testing? CCing Myles as the last person to touch this ;)
<rdar://problem/70353187>
This works for me now.