Hurz
Use of the national language in aviation isn't confined to South America, it's quite common over Europe and Asia too.
AFAIK English is used by Portuguese speaking crews over Spanish speaking territory, and vice-versa.
The overall (don't bite, I know there are many pilots and controllers who speak fluent English) standard of English in the worldwide aviation community is fairly low, often just enough to satisfy the minimum requirements, and so the use of a national language improves situational awareness for many crews, who are in the majority in national airspace.
I agree that this reduces the awareness for foreign crews, and I agree that English, as the international language of aviation, should be used at all times. However, until the overall standards are improved situational awareness for the majority is aided by the use of the national language.
alemaobaiano