So I'll volonteer my observations. It's not oncommon to hear English words in contemporary songs because the contemporary song makers speak English.
Many singers know little or none of their traditional tribal languages.
There is a move afoot amongst many tribes to teach and preserve their languages before they are lost completely. You don't much English in the old songs but then many of the old songs are now sung with vocables because people of many tribes and language backgrounds are singing them the vocabls are easier. It can be hard to even find someone who knows the original words or meanings. It is merely my observation that at this time the norther singers are more likely to be creating new songs and therefore more likely to have songs with English words. Seems like the Southern drums are still more likely to be singing older songs.