That's my favorite subject

(just kidding).... unfortunately, all these comments are real (the good and bad ones). Everything depends on the tour group.
If you assume that every Brazilian teenager in WDW is able to talk in English fluently, you are absolutely wrong.
If you assume that a BTG is composed by upper class teens, you are redundantly wrong, middle class at most. Usually, Brazilian upper class would not use tour group services (not these clearly working in WDW).
Anyway, don't get me wrong, I'm Brazilian and the BTGs are really annoying (as well as any other unmannered tour group).
As Brazilian, I can see many details on a BTG, to guess the level of "politeness".
Ok.... it's not discriminatory... just a hint... considering my country and the intrinsic details (multi-cultural federation, economy at each region/state, education level, etc), try to see what's the Brazilian state/region... usually is specified by two letters (e.g: Brazil / CE, Brazil / MG)....
We (my family and I) learned on the hard way... but to avoid discrimination I will not mention any Brazilian region/state.
There are specific really bad tour groups, all of them "specialized" in teenagers... we avoid them independently the origin / region.
Of course, there are many regular (good, I meant) BTGs, all of them are official Disney travel agencies (RCA, Agaxtour, just to mention two)...
Coincidence or not, we only had problems with those non-"Disney Select Tour Operator".
So, generalization will not work and there is no easy answer...
We are going again for Christmas/New Year's Eve & Marathon... (dec/12 - Jan14), the ideia is avoid "X"TGs and have good time (not an easy task at this period of the year).