Some locales might wish to format numbers using either local digits or ASCII digits. The grouping and decimal symbols used with them will be different, not merely the zero digit or algorithm. The symbols used with ASCII digits will also differ by locale, as they do between, say, the U.S. and France.
This should be representable as a locale extension on the locale id. I expect we don't need more values than 'local' and 'ascii', so supporting only two sets is probably sufficient.
The default might vary among locales that support a local option, so we'll need data indicating the default to use when no decimal system is specified with a locale.
NumberFormat or DecimalFormat will want API to get/set this information as well.
Use numbering system id instead of 'local' and 'ASCII'
Need ability to return the numbering systems available for a locale.
John has implemented this under other tickets.