That's very true, of course, but it is possible to have a character's voice down in most respects and yet use the occasional word in a sense in which it isn't used in his country, or to use words that aren't location-appropriate in the narrative (which might not be written to exactly mimic any one character's voice). Jack O'Neill talking like Jeremy Clarkson would probably bother me more than Jack O'Neill saying 'aeroplane', though.