libusb: implement libusb_setlocale to support different locale of error message

Upstream libusb identifies languages using only the first two
characters of the language code.
We adopt the same approach to maintain API compatibility.

Currently, only English and Mandarin are supported,
as other languages are not yet implemented.

Reviewed by: kevans
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50820