new Intl.RelativeTimeFormat("en", {numberingSystem:"hanidec"}).format(2, "days") is currently spec'ed to return the string "in 2 days" instead of "in 二 days".

new Intl.RelativeTimeFormat("en-u-nu-hanidec").format(2, "days") is correctly spec'ed to return "in 二 days", though.

InitializeRelativeTimeFormat

19. Let relativeTimeFormat.[[NumberFormat]] be ! Construct(%NumberFormat%, « locale »).

Should instead be:

19. Let nfOptions be OrdinaryObjectCreate(null).
20. Perform ! CreateDataPropertyOrThrow(nfOptions, "numberingSystem", relativeTimeFormat.[[NumberingSystem]]).
21. Let relativeTimeFormat.[[NumberFormat]] be ! Construct(%NumberFormat%, « locale, nfOptions »).

Also applies to the Intl.NumberFormat instances in FormatDateTimePattern.