localization improvements

[ray007]

It would be nice, if strftime.localizeByIdentifier() also worked with just a 2-character language code, i.e. "de" instead of "de_DE".

And maybe something so we don't have to specify the difference in the formats to the default locale for strftime.localize()...

if Intl.DateTimeFormat is present, it should also be possible to generate a locale. At least the basics, the locale dependent formatters are a bit tricky, but we could fall back on the default locale there.

I can help out with some of this if you're interested.