In other words, Tonto could utter complex as well as simple thoughts. Most movies use broken English to show civilized white men can think and talk better than savage Indians. To signify that Indians aren't capable of creative or independent thought.
When you hear movie Indians talking like this, repeatedly, that isn't just broken English. It's a ridiculous, exaggerated version of broken English that nobody ever spoke. It doesn't indicate that the speaker doesn't know English. It indicates that he's a buffoonish cartoon character that we shouldn't take seriously.
Really, how long would it take an Indian listening to English to realize that the correct pronoun is "I," not "me," and that there's no such suffix as "-um"? A toddler would learn this in a few weeks. Portraying Indians as people who can't grasp basic English after months or years of exposure is meant to denigrate them. It's insulting, not accurate.
So no, there's no excuse for the Tonto talk heard in many Westerns. People who don't speak English 1) speak their own language fluently, 2) express complex thoughts in whatever language they speak, and 3) don't make the same grammatical mistakes over and over. A movie that shows Indians talking like Tonto is almost certainly wrong. It's repeating decades of stereotyping, not depicting genuine ESL speakers.
For more on Studi, see Runningwater Interviews Studi and Studi Challenges Stereotypical Roles.
Below: The type of Indians who almost always speak broken English. It's no coincidence that their cartoonish speech matches their cartoonish appearances.