Originally Posted by Steven Lake
Mr. Ploppy has it right. Educated or not, there are language idioms, colloquialisms, verbal shorthand that define people from a particular region or nationality and are imperative for getting the character's voice right. People from other countries also quite often have different names for things. For example, to the British, a flashlight is a "torch", a truck is a "lorry", and a highway is a "carriage way". Or take cars for example. To them a hood is a "bonnet", and the truck is the "boot". Again, to use another example of localised word use, you can't expect a Canadian to say they're from "out yonder", but you can certainly expect someone from down south to say that. There's also the mannerisms, what they consider polite, rude, in some cases curse words, general expressions, etc. In short, to get a character of a defined nationality right, you have to ensure that all of the obvious expressions are visible to the reader. Ie, the things they expect. If you don't have it, you kill the story by shattering the ability to suspend belief.
Just to emphasise how difficult it can be, you wouldn't very often hear somebody British describe a "highway" as a "carriageway" actually. It would just be a "road". The only time you would invariably hear "carriageway" would be as part of the term "dual carriageway", used to describe a road with two lanes going in each direction.