On the specific point of:

>   2.1 some languages/alphabets use other chars (e.g. a comma or other
>       symbols) instead of the decimal point.

I think it's not the job of the float() constructor to support it.
Depending on the country, the comma has different meanings when put in a
number (thousands separator or decimal separator). Ditto for the point,
but using a point as decimal separator is the accepted standard for
non-localized computer I/O.

More generally, I think the fact that int(), float() et al. support
non-ASCII decimal digits should be seen as a convenience rather than a
willingness to accomodate the broadest set possible of inputs. Which
means, we should only add support for new formats only if it's sensible,
safe and non-ambiguous to do so.

I also agree with Marc-André's argument that the Unicode spec should be
a good guide here.