These are various Mule-related files, some modified from the Emacs 21.1 sources for various purposes, others new. They're mostly whole source files (which I've been using to some extent) rather than diffs which are more of a pain to manage. It's probably all Emacs 21-specific.

lisp/international/characters.el.gz
Extended, particularly for unicodes.
lisp/international/ucs-tables.el.gz
Translation between Unicode and other Mule charsets, providing unification of European charsets (unify-8859-on-decoding-mode and unify-8859-on-encoding-mode). New command ucs-insert. Extended decode-char, encode-char. Hook into Quail to translate input method characters conformant with buffer file coding system. Extended ccl-encode-unicode-font to be able to display characters in characters in the tables with an iso10646 font (via set-fontset-font).
lisp/international/utf-16.el.gz
New file for UTF-16 coding systems.
lisp/international/utf-8-subst.el.gz
New file for use by utf-8.el with CJK.
lisp/international/utf-8.el.gz
Changes:
lisp/international/utf-7.el.gz
New file for UTF-7 coding system. Requires utf-16.
lisp/international/code-pages.el.gz
New file with many extra Unicode-based 8-bit coding systems and a macro to build them. Should replace codepage.el.
lisp/international/mule-diag.el.gz
Various changes to provide more information, e.g. about Unicodes, and support for code-pages.el.
lisp/international/latin1-disp.el.gz
Extended, especially for Unicode.
lisp/international/mule-cmds.el.gz
Mainly extended/corrected processing of locale specifications.
lisp/international/quail.el.gz
Mainly provide translation of input through standard-translation-table-for-input so method coding can be made to conform to buffer's.
lisp/language/cyrillic.el.gz
Reworked. Complete and correct koi8-r and alternativj. Add koi8-u. Add unification between ISO 8859-5 and Unicode (including recoding for fonts). Various new language modes (depending on ucs-tables.el, code-pages.el, Quail changes).
lisp/language/cyril-util.el
Fixed and extended.
lisp/language/georgian.el
New environment.
lisp/language/european.el
Extra environments. Support for composition of diacritics.
lisp/language/utf-8-lang.el
utf-8 pseudo-language environment, auto file decoding for .utf.
leim/quail/cyrillic.el
Somewhat reworked and variously extended.
leim/quail/latin-ltx.el
Extended/corrected.
leim/quail/sgml-input.el
leim/quail/uni-input.el
leim/quail/rfc1345.el
leim/quail/georgian.el
leim/quail/welsh.el
New input methods.
lisp/language/lao.el
lisp/language/lao-util.el
lisp/language/thai.el
lisp/language/thai-util.el
Support for Unicode composition.
trans.el
Multibyte character translation and transcoding.
ucp.el
Finding un-encodable characters. Potentially-useful for finding encoding boundaries, e.g. multiple charset regions in Gnus MIME encoding, or simply finding an errant character with things like select-safe-coding-system.
loadup.el.diff
Change to loadup.el to allow dumping the utf-8/utf-8-subst/characters/ucs-tables combination. (decode-char shouldn't really be in ucs-tables.)
mule-ucs-0.84.diff
Fixes to Mule-UCS for Lao and Devanagari and to significantly speed up loading.