Yes, this interests me greatly, too...
Good it has been brought up for discussion.
Amazing to find so many different opinions, each supported by the author �s experience.
I don �t speak my students � languages (Creole/French and Portugues, and recently Spanish).
(Although I have some knowledge of French, and I generally pick up languages easily, I can �t help understanding what Brazilians are talking about only from the few words that are common for Latinic languages and their intonation.)
As said above, it does depend on many factors.
We should differentiate perhaps between two main situations:
one in which the teacher knows the students � language
and
two in which the teacher doesn �t know the students � language.
As I said, I don �t speak their languages, but yet I like them to make a bridge between their language and English and thus dive into a more linguistic view of what they are studying.
I sometimes give them translation activities, in which they speak in their language for a few minutes during the class. I allow them to explain things to each other, but only the things taht are the concern of the class topic.
We translate grammar explanations, and I always have good feedback, students say that points become clearer, for instance we were covering the Passive the other day, and both Brazilians and Mauritians told me that in their languages it was impossible to make a passive sentence with a human object. I was happy that they made this observation, because I believe that really knowing English is possible only once the students knows his/her language. My weaker students appreciate this, too, I see genuine interest in their eyes and I see that they understand what important grammar points I want them to understand.
When we have vocabulary test, I often ask hem to translate the word and then make a sentence in English, thus I ensure that they really do understand.
In the break time they compensate by chatting amongst themselves in... guess what? ... yes, in English! :)
Apart from that, they come across English on daily basis in their lives, they hear it, they read it, they speak it, so I suppose learning a language in English-speaking country carries less risks than perhaps learning it in the country where they are from. (If I taught English in their country I think I would be very strict with the amount of time that English is spoken during the class.)
I believe that making a connection between the two languages helps students to appreciate their language, appreciate English, appreciate their own intelligence and wits, make them feel good about their language and thus about their cultures and countries (often poorer and less developed than Ireland), and about the study process, too.