At the part when they are in the gym discussing their problems. They go to the Asians and the girls are really talking in Vietnamese. Skip towards the end when they are in their new cliques, Gretchen is speaking to the Asian girls. What she is actually doing is saying words, she isn't really speaking Vietnamese.
Do you actually think a stupid b*tch like gretchen could learn vietnamese in a few weeks?! doubt it.
Also, we have seen the extents she goes to to blend in. I think this is just another thing to make her feel like she belongs.