The following proposal allows the calendar and numberingSystem to be set in the options bag for Intl objects: https://github.com/tc39/ecma402/pull/175 V8 and SpiderMonkey have implementations of this behavior ready to go. Until WebKit adds support, users will get poorer i18n support relative to other browsers.
<rdar://problem/61079801>
Created attachment 396203 [details] Patch
Created attachment 396209 [details] Patch
Committed r259941: <https://trac.webkit.org/changeset/259941> All reviewed patches have been landed. Closing bug and clearing flags on attachment 396209 [details].