Working in an internationalization proposal <http://python.org.ar/pyar/TracebackInternationalizationProposal>
I've stopped at #9769 where multi byte encodings (like utf-8) is not supported by PyUnicode_FromFormatV()

Beside my proposal, I think utf-8 should be supported for consistency with the other unicode functions, like PyUnicode_FromString() or even unicode_fromformat_arg()

Attached is a patch that:
- enhanced the iterator to detect multibyte sequences, with sanity checks about start & continuation bytes
- replaced unicode_write_cstr with PyUnicode_DecodeUTF8Stateful
- tests

Hope it helps, this is my first patch for cpython and my C skills are a bit rusty, so excuse me if there is any newbie glitch