Spanish is the official language of Chile. As you may already be aware, in informal contexts Chilean speak a dialect of Spanish that differs from that which you may have learnt or heard elsewhere. If you can go about in conventional Spanish, most Chileans will be happy to oblige in a standardised, more intelligible speech, but even their accent can be dismayingly difficult to understand! If you have no choice but to default to English, younger generations tend to have more knowledge of the language than the elderly. Avoid, however, starting a conversation in English assuming that the person you are addressing knows or should know English. This will inevitably come across as rude. Kicking off with buenos días, no hablo español will get you further than you think.