> '1?2:3?4:5' -> '(2 if 1 else 3)?4:5' -> '(4 if (2 if 1 else 3) else 5'

This is not right. It's right associative so it should be

1?2:(3?4:5) -> 1?2:(4 if 3 else 5) -> 2 if 1 else (4 if 3 else 5)

> It would be nice to make c2py() working with any expressions, but if this is too hard, this can be left for other issue.

Agree. But I am interested in trying.

> gettext_c2py.patch itself LGTM for fixing security issue, but tests are needed.

It gets drawbacks so I don't include tests. I'll add in next try.