support more English (and French) locales by default

[juhp]

I reckon format files should work fairly well across locales for the same language.

How about just renaming en_US.json to en.json and similarly for fr_FR.json?

Otherwise it only works for en_US and fr_FR locales I think - which seems overly restrictive

(cc @Manish7093)

[juhp]

I suppose more specific locale files could still be supported but falling back to the generic one if not defined.
That would be a kinder approach IMHO.