One of the first things to understand was how people knew what language to speak to whom. Where I've lived in the American Southwest, choosing to speak English or Spanish based on how someone looks is risky. If you try English and they don't speak it, you can switch to Spanish if you know it. But if you start with Spanish, you might offend: 'You don't think I speak English?' This can be the case if you're Anglo, even if you speak Spanish very well and just heard the other person speaking Spanish. When I described such an scenario to Indians, they couldn't relate — to them, choosing the wrong language wasn't embarrassing or politically charged. Or so they said.
Michael Erard, Babel No More: The Search for the World's Most Extraordinary Language Learners
language, multilingualism