I get this question all the time. It happens like this:
You: "What's your job?"
Me: "I'm a teacher."
You: "So what subject do you teach?"
Me: "English... as a second language."
You: "What other languages do you speak?"
There it is. The most frequently asked question. It happens nearly every time I talk about being an ESL (English as a Second Language) teacher.
I teach in English
First of all, it would be impossible to speak all the languages represented in my classroom. It's super diverse, 5-10 languages and cultures often. How can I possibly speak Arabic, Persian, Chinese, Japanese, Spanish, German, and French all at the same time while I'm teaching? hmmm... Secondly, research has shown that it's actually better to teach a language, like English, without knowing your students' first language. (What?!) I know, weird. But it's true. I'll get to that more later on.
Some ESL teachers, yes, speak another language. I'm not saying that you shouldn't to be an ESL teacher. I just don't. (I tried Spanish, but it didn't stick.) I often wish I knew all the languages spoken in my classroom. (Then I could hear if they're cheating or what they really think about me as a teacher!)
My best friend speaks Mandarin Chinese as her first language, and English as her second. She's teaching in China right now and her Chinese has pros and cons for her teaching. My friend Mike learned English first and than Japanese. Another friend, Sarah, speaks English first and then Korean. Others of my teaching-buddies are like me and speak English only.
The essential requirement for teaching English, it seems, is knowing English. And lots of schooling. Lots! All of the people I mentioned have at least a Master's degree, which is what I have, too. So before you ask the question of whether or not native speakers are better teachers than those who are ESL, let me tell you that my best friend is a better ESL teacher than I could ever hope to be. She may not be blonde and blue-eyed like me, but she knows what she's doing! Just because you grew up learning a language doesn't mean you can teach it.
How we learn languages
There's a lot of research about how we learn languages! I'll try not to bore you with the science of linguistics. Instead, I'll invite you to think about how you learned your first language. Probably when you were a baby, yes? Chances are that your motivation for learning (which is key to learning a language, by the way!) was out of necessity. You wanted your caregiver to know you were hungry or that you wanted a toy. Crying and screaming did the trick for some things you wanted, but not everything. Eventually you had to learn words and sentences to get what you needed. Then, there's the social necessity. You needed to communicate with your friends, too. Generally speaking, this is a similar process for learning a second (or third language) effectively. There must be a necessity for it. (Whether that's feigned or not.)
Can adults learn languages?
There's the "Critical Period Hypothesis," yes, which I figured you'd want to know next. Lenneberg proposed the theory in 1967, basically saying that kids must be exposed to a second language before puberty (generally 13 years old). Otherwise, it would be nearly impossible for that person to learn a second language.
There is truth to this, to some degree, but not entirely. Otherwise, I'd be out of a job. All of my students are adults. I've come to learn that it is difficult for adults to learn a language but not because they're brains are fully formed and not as moldable as a child's. (That's part of the more modern version of the "Critical Period Hypothesis.") Simply stated, the necessity to learn a second language isn't as high for an adults because they know a first language that can usually get them by. (This is why I never fully learned Spanish, by the way.) Also, there are other responsibilities that kids don't have to think about which puts adults at a disadvantage: paying bills, choosing a place to live, finding a job, etc. These things could potentially give necessity for learning a second language, but more often than not, it takes adults away from practicing and perfecting a second language.
There's a lot more science and research behind all this. If you're really interested, you can earn a Linguistics degree or an MTESOL (Master's in Teaching English to Speakers of Other Languages.) I did the latter. Quick advice from me if you want to learn a second language? Be super motivated, disciplined and don't be afraid to make mistakes; if possible, immerse yourself in the language!
How I teach ESL using only English
Here's where we finally get to the heart of your question: how I teach English without knowing any of my students' first language. There are a lot of theories and practices out there! I'm not going to pretend there's a one-fit-all solution. (By the way, Rosetta Stone is only effective for probably 1% of the human population. We're not robots!)
I have a teaching philosophy statement, which I framed and put above my office desk. It includes things like building a safe classroom environment, instilling student autonomy and giving timely and individualized feedback. In general, however, my teaching varies in two ways, whether I'm teaching beginners or I'm teaching intermediate/advanced speakers of English.
When I teach basic students (or beginners), my teaching is a game of charades:
When I teach intermediate or advanced speakers of English, I build motivation (the necessity factor, I mentioned), and create (for lack of better wording) a-close-to-real-world-experience:
Teaching ESL is a great profession. It's difficult, don't get me wrong! There are days when I want to pull my hair out. (That's true for any teaching job, though!) But I love it! I learn a lot from my students. They bring interesting and new perspectives, they're motivated and have worthwhile goals. I admire the sacrifices they're making, coming to a new country to have a better life for themselves and their families. English is a crazy mutt language, and I commend for their persistence and hard work!
So, yep. I'm an ESL teacher. I teach English... in English.