My suggestion would be first to have one of your colleagues inquire before they left to see if they Chinese embassy on Conn Ave can hook them up. Or you may inquire at:
2300 CONNECTICUT AVENUE, N.W.,
WASHINGTON D.C. 20008, U.S.A.
Tel: 202-328-2500

For general tours of DC(monuments, Capital, etc) the popular Tourmobile is English only. You may want inquire here about private tours since I don't think you're going to get much luck on a walk-up basis:

http://www.washingtondctourguides.com/guides/index.html

The Smithsonian has foreign language tours for certain museums and I would highly recommend either one of these as they focus on art from the Far East:

http://www.si.edu/events/tours/fga.htm

http://www.si.edu/events/tours/sga.htm

The Smithsonian Art Museums are really a hidden treasure. They provide a nice break from the Mall's tourist atmosphere in any language!

Edit:Oh...and found this with a google search:

http://chinese-school.netfirms.com/C...ington-DC.html