how would you help promote multiculturality in an ESL classroom ?
It depends upon what cultures are involved. One way is to use names common in other countries when having students repeat sentences. For example, don't always use "John" or "Susan" when giving examples. Use names common in the countries from which your students hail.
You can also present lessons about holidays that you might not necessarily celebrate, but your students do, such as Cinco de Mayo, or Chinese New Year, or Ramadan.