This question has been a staple of language teachers and learners everywhere. The simple answer is: The languge you think in is the best medium to teach any other language. If someone is going to learn the language by constant exposure to it, he does not need any teacher at all.
In the class room environs, however, it is the mother tongue/the language used in thought process that SHOULD be used to teach English. This is because, the student already knows the appropriate word for each thing or action and all the components of a sentence. For instance, he knows palm as Ullangai. The student always had identified the palm as ullangai. For him to remember the word palm, he would always relate it to ullangai. This is how it would be with all NEW words which obviously would be in their thousands.
You have a limited time of 6 months or a year to teach reasonably good English. You can not keep concentrating on one word for hours together. It should be taught incidentally and you need to move on.
I have been teaching English independently, sans the burden of a syllabic boundary for 30 years. 1000s of my students speak perfect English (of course with a marked Tamil accent). Watch my 10 videos at youtube.
http://www.youtube.com/watch?v=Uryl7T-QBUQThank you