You Don't Need to
It's not necessary to speak the local language in order to get a job teaching English. Although some employers require you to know the language, they're few and far between. More often than not they'll forbid you to use the students' L1 in class unless absolutely necessary. After all, they're paying you to speak English, not their language. Although it's nice to be able to understand what your students are saying, if you don't know the language, it's not the end of the world.
Outside the classroom is a completely different story. In some countries, it's pretty easy to get by without knowing the language. Signs may be in English and people might know English. However, in other countries it's more difficult. If you're in one of those countries where English isn't used and you don't speak the local language you'll probably do a couple things. You'll learn a bit of the language, rely on signing and body language, or befriend a local. If that sounds ok with you and if you're ready to make the move then try reading, 30 Days to Move Abroad.
Living Abroad Doesn't Mean Fluency
Let's face it, the simple fact that you're going abroad to teach English means that you're not really immersed in the language, are you? Your boss speaks to you in English, as do your co-workers, students, and many of your friends. Just like sleeping with your book under your pillow doesn't mean you'll learn anything, going abroad isn't going to guarantee that you'll learn the language.
There are many ways to learn a foreign language. I've written a post about how to improve your English and the tips in there hold true for learning a foreign language as well. Keep in mind that some languages are harder to learn than others. Some people learn languages quickly, like Benny the Irish polyglot. If you're interested in learning the local language, then try reading learning the local lingo and learning the local lingo while teaching ESL.
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