> I haven't looked at this closely yet, but you'll need to at least:
> - add tests that the locale-aware formatting is happening

Done.

> - support decimal
> - make sure it works with complex

Good points. Done. Please note, that there is an inconsistency between float/complex/int/_pydecimal(!) and decimal. The former provide only 'n' format type and the latter provides 'n' and 'N'. So I implemented 'm' and 'M' for decimal and 'm' for _pydecimal.

> (which it probably does, but needs a test)

There are no tests for 'n'. Should I create for both 'm' and 'n'?

> And, I think we'll need to run this through python-ideas first. One thing I expect to come up there: why f and not g?

Because 'g' has been already covered with 'n'.