Using internet archives, forums, wikis, and Usenet posts, I can establish the following about Jack:
Claimed proficiency in 1996: C, C , Perl, Tcl/Tk, Java, NewtonScript, x86 asm, ARM 610 asm
Claimed proficiency in 1999: C, Python, OCaml, JoCaml, Lisp, ObjC, Java, Perl, PHP
Jack Dorsey shows up in various programming forums including for Gentoo, Microsoft Win32, Java, Python, and more.
Jack's record show him using various flavors of Linux, OpenBSD & FreeBSD, Windows, BlackBerry OS, and Mac.
He shows up on the cypherpunk mailing list from at least 1996 - 2001, alt. security.pgp in 1993, sci.crypt in 1999
I have only found samples of his Python and Objective C code so far and it's on the site for record keeping.
The skeptic says "show me the C " and that's reasonable. While it's long stated that C in particular has been his favorite language going back to the 90s, even that does not appear to have been for open source software that is discoverable on the web aside from these very few lines in June 2001 that he was writing for his father's spectrometry company:
static const handler_rec mga_handlers[] = {
{ "mga", mga_handler },
{ NULL, NULL }
};
So I am still on the hunt for his C and C code. Its stated on his website in the 90s that he programmed in them, but I don't know if samples are discoverable.