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

Shouldn't this patch be attached to the referenced issue 9769 instead of creating a new issue?  Even the issue title is nearly the same:

9769:  PyUnicode_FromFormatV() doesn't handle non-ascii text correctly
16343: PyUnicode_FromFormatV() doesn't support utf-8 text

I thought #9769 was closed (in fact, that patch was already applied).
Now, PyUnicode_FromFormatV() doesn't handle non-ascii text at all.
Maybe I misread the part telling to open a new issue in the comments, sorry for that.

Issue 9769 is still open.  It looks like there was some disagreement in the comments between Alexander and Victor as to whether a new issue should be created (since Victor had a different idea when first opening the issue), but it looks like Victor deferred to Alexander in his final comment.

I will close this issue as a duplicate, and if you could repost your patch on the other issue, that would be great.  The discussion there is relevant to your patch.