All tour guides are from their home country, AFAIK.
Here in Brazil most tour guides have a degree in Tourism and Hospitality (I know, the irony!) and must speak fluent English to be hired. Some of them have extensive knowledge of Disney, they read up on attractions, best time to visit the parks, recommended park; everything you and I do for our trip, they do for their groups.
The job of tour guide is to make the group's visit perfect. They are rated on their work when the group returns and will be evaluated after a period and you know how kids don't judge a job by how knowledgeable and efficient it's done but in how much fun they had, so I guess that's why tour guides will overlook most obnoxious behaviors we consider rude.
I suspect the "very fluent" requirement is defined by each business.... from my own experience in dealing with Brasilians that were "very fluent" it was all a matter of who was deciding what very fluent was. In law firms the typical person fluent in English was truly fluent to the point where you could drop them off in the middle of the US or England and they would be perfectly fine... Compared to a local tour company we used in Rio that was supposed to have "very fluent" guides for some folks we had visiting that were anything but, their fluency was in nodding, saying yes to whatever question was asked and then reciting some memorized English lines as they took the guest around...
But what I have noticed that is missing in the few Brasilian tour groups I've run across is any of the so called chaperons, unless they are the same age as the kids I haven't seen any... unless the kids had already taken them prisoner and tied them up somewhere before taking off... unless that is the secret to getting high grades from the kids - stay as far away from your group as possible.