I agree that it would be great to have such a facility. I'm not sure we need locale:, we could just allow it as a second parameter (once we have made sure that there are no other suitable candidates for further parameters). I would personally start with 'output' functions (i.e. to_s), because they are way more deterministic.

Please note that currently, Float("4,294,967,295.0") throws an error, we may want (or not) to keep it that way, and only allow commas if a locale is explicitly set.

Also, we may want to create some kind of Locale object and allow this as a parameter. Also, we may want to allow more direct specification (e.g. decimal_separator: ',', thousands_separator: '.'; but note that this isn't complete because in other locales (e.g. India), the separator will not separate thousands).