It's good that we're distinguishing between lexical types and datatypes.
With real datatypes, it seems that we ought to be able to hard-wire
the syntax - i.e. say that for FIXED or FLOAT data types, the following
decimal delimiter is always used, for DATE it's always ISO8601, and
so on. The idea being that this kind of thing, in a production type
app, is going to be run through software machinery at both ends, such
that on the screen, a Francophone will see 4,50 and and Anglophone 4.50,
so it doesn't matter that much what's between the tags.
It seems to me that this kind of hardwired datatyping is in greater
short-term demand and is also easier to implement efficiently than
lexical typing, thus that's where I'd start; but reasonable people
may disagree. -Tim