The official language of Chile is Spanish, and few Chileans outside of the tourism industry speak more than rudimentary English -- so bone up on a few handy phrases before arriving. Chileans appreciate the effort, and really, part of the fun of traveling is learning the local lingo.
That said, even Spanish speakers have a difficult time understanding singsong, high-pitched Chilean Spanish, which has grown to be known as chilensis for its rapid-fire delivery and heavy use of local phrases and slang. The most notable peculiarity about Chilean Spanish is the merge of the formal vosotros with the casual tu verb forms, which over the centuries has created a verb tense unique to this country. Chileans use "tu estas," or "tu comes," but it's very common to hear instead "tu estai" or "tu comai." This -ai ending is used in very informal settings; most popular is the greeting, "¿Como estai?" Another oddity in Chilean Spanish is "pues," which puts emphasis on a word, and is more commonly shortened to "poh," as in "Sí, poh," meaning "Well yes!" Words that end in -ado or -ido typically drop the "d," so that pelado becomes "pelao." Chileans also drop the "s" in words, so that más becomes "ma."
While some Latin countries such as Argentina have virtually dropped the usted verb form except in the most formal of occasions, Chileans use the usted form habitually. Waiters, doormen, strangers, and any new business associate should be greeted with usted until you become better acquainted.
