How to teach your kid you mother tongue when you live permanently abroad? When is the right time to do so and is it a good idea?

On my experience as teacher is different as parent can say. But remember that children mind are like sponges.. every knowledge they can take it very easy...

The age for start doing that is between2 and3 years old.. once i teach italian to a baby of3 years old, and now is a10 year old kid that speaks perfectly Spanish, English (our second language in mexico) and italian.

In my case, my mother language is Spanish, second English... my fiancé mother language Arab and second French... so for our kids we decide first mother language Arab at same time Spanish... then the english and french.

The benefits of teacher mother tongue according to most literature is that it is an excellent idea.  The child at a young age can pick up other languages easily.  At home the focus should be on the mother tongue.  Eventually this will help in language acquisition.

It is a difficult question Samar, but I believe that teaching my son his mother tongue can be done on a daily basis through repetition and familiarization with the mother tongue's sounds

I agree with Yasmine, I live in the UK but made it a point to only speak our mother tongue with our kids and this was also recommended by the teachers at school. kids are able to pick up any language as a native user before the age of puberty so its not really a problem how many languages you put in front of them, chances are they will develop them very quickly. 

Of course it's a good idea and you have to start as soon and early in his age as possible. Don't worry, your kid will acquire it naturally when you make it a habit as a family to always speak the mother tongue at home. This way he will normally speak the foreign language at school and with his friends, and the mother tongue at home and with his family members.   

Salam alaykom, hi everybody ,i'm a teacher of english in Algeria , i would like to tell you that the kid  has a powerful skills , don't worry , according to scientists, the first years of childhood are the best period to get a foreign language , if you are really interested chech the theory of PIAGET

First of all, we have to watch ourselves. I noticed that most of us switch between Arabic and English when speaking with each other. I remember an ad on Youtube that said something in Arabic then said "don't skip."

It's better to think of a way to eliminate this problem. We can try to tell them what we say in Arabic means in English and vice versa.