RFCs (requests for comment on Internet draft standards) are available on most large archive sites and are packaged as optional documentation by some Linux distributions. A primary source for RFCs and Internet standards is the Information Services Institute at the University of Southern California, http://www.isi.edu/; they manage the Internet Assigned Numbers Authority and distribution of RFCs, among other services.
The resources below may be useful to developers wishing to internationalize their programs or users trying to get the most out of their localized software. This list is far from complete and mostly reflects what I have looked at recently.
POSIX: the portable operating system interface standard (not available on-line, unfortunately)
POSIX locales: locale definitions are collected by Keld Simonsen for the WG15 working group. These are available for FTP from ftp://dkuug.dk/i18n/WG15-collection/; a fair amount of I18N material is on that server. If you understand Danish, there is also a web server at http://www.dkuug.dk/.
Mail and news: RFC 821, RFC 822, RFC 1036, RFC 1123
MIME: RFC-2045, RFC-2046, RFC-2047, RFC-2048, and RFC-2049, especially 2045 on format of message bodies, 2047 on encoding of non-ASCII text in headers and the updates in RFC-2231
MIME charsets: RFC-1700 (ancient); most recent version is available from http://www.isi.edu/in-notes/iana/assignments/character-sets/.
UTF-8: RFC-2044, also the appendices to Unicode and ISO 10646
Unicode: The Unicode Standard, Version 2.0, softcover book; try your favorite on-line bookstore.
ISO 646, ISO 2022, ISO 8859, ISO 10646: The ISO standards are available directly from the International Standards Organization—not terribly interesting. For ISO 10646, get the Unicode book instead as it is less expensive and more informative, except that it does not cover UCS-4. Most of the information pertaining to the ISO-8859 series is in the ISO 10646 and Unicode standards.
GNU gettext: Info documentation for gettext (often libi18n on Linux systems)
Asian languages: Understanding Japanese Information Processing, Ken Lunde, O'Reilly and Associates; a greatly expanded edition, covering Chinese, Korean and Vietnamese as well as Japanese, is in press.
Corrections to this article and resource updates will be available at http://turnbull.sk.tsukuba.ac.jp/Tools/I18N/.