The bottom line is that it is not necessary to have knowledge of any language other than English when taking a TESOL course or when teaching English abroad. In the modern age, the majority of English classes around the world are based on the communicative approach to English teaching that simply means that English is the only language used in the classroom by the teacher and the students.
How can I teach English if I don't speak the student's language?
You may find yourself in a situation where you have a multilingual classroom that contains speakers of a dozen or more different languages. In these scenarios being able to speak one or two of them would be unlikely to help anyone. In contrast, teachers in monolingual classrooms often find that their students are more likely to revert to their common language during a lesson if they know that the teacher can understand them. Whether you are in a multilingual or monolingual environment, the main principle of ESL teaching is that the classroom only communicates in English, from the moment the class starts until the very last minute.
Does it help if a teacher has learnt a foreign language?
Although you do not need to speak any other language to be a great ESL teacher, you might be at an advantage if you have some experience in learning or using languages other than English. Any experience learning another language should help you empathize with what your student's are feeling and help you to improve your approach to teaching. Having foreign language experience can also lead to a greater understanding of the structure of language in areas such as grammar, etc.
Should I learn the local language while teaching English abroad?
Many teachers living and working abroad will naturally pick up a certain amount of the local language during their stay. The ability to communicate, even on a basic level, can have a major impact on your experience of being overseas and make it much easier to complete small interactions on a daily basis. Some people choose to take it further and have regular lessons in an effort to become fluent in the language of their adopted country. However, as long as you are equipped with the skills and knowledge that a TESOL course provides, it is not in anyway essential to have knowledge of your student?s native language.