possibly during harvest which was before Halo: CE they only spoke well alien -.-" or even before Harvest i kinda think during harvest they were on patroll a buch of elites and found some marines caught them and interogated them didnt understand them much used translaters but decited to speak a little like them possible spy's were only taught to speak english but thenthey taught all of them to speak english backwards on halo: CE or another halo game cortana ses they spoke english but backwards so he could say to a marine (right way) "i'm going to kill you f*****" (oposti way) "ckeruf uoy llik ot gniog m'i"  strange really strange marine would think "your going to hug me???"