The simple answer to this question is you do not need to have knowledge of any language other than English to take a TEFL course or to teach English anywhere in the world. This is possible because the modern day language classroom typically follows the communicative approach which simply means that the teacher and students only use English throughout the entire lesson.
How do I teach English without knowing the student's own language?
Many teachers find themselves working with a multilingual class that contains students who speak a wide range of different languages. In this common situation it is not possible for the teacher to be familiar with all the languages present in the room. Equally, if you are teaching in a monolingual classroom it is common for the students to fall back on their native language more often if they are aware that the teacher can understand what they are saying. Regardless of which scenario you end up teaching in, you should always stick to the first principle of EFL teaching, that English is the sole language used from start to finish of the class.
Will it help me if I have learnt a foreign language in the past?
While it is unnecessary to speak any other languages and in many cases counterproductive to use them in the classroom, it can help the teacher to empathize with their students if they have experienced learning or using other languages in their own life. Learning a second language can certainly improve your understanding of the teaching process and lead to a wider knowledge of things such as grammar structures. By using this experience when planning and presenting your own lessons you can potentially gain better results in the classroom.
Is it worth learning the local language when teaching English overseas?
If you head overseas to teach in a non-English speaking country it is likely that you will pick up a fair amount of the local language naturally during your stay. Having the ability to communicate to some degree with the local people can certainly enhance the overall experience of living and working abroad and can lead to new friendships and unique adventures. If you want to take it a step further there is nothing stopping you taking regular language lessons as you work towards greater fluency in your new home country. However, this is all personal choice as all you really need to start your teaching career is a TEFL certification and a desire to explore new horizons.