My children and dh are trilingual and I know a lot of trilingual families in RL.
I speak English to the children, dh in French and they're in a bilingual French-German program at school. We're in France but near Germany. Dh also speaks German and the German dialect spoken here but he's strongest in French. He was raised bilingually in French and the dialect. He doesn't speak English but after 10 years, understands it pretty well, listening to me with the kids.
You actually have an ideal trilingual situation. Your dh might want to slowly shift to Persian so to make it easier on his mom. Otherwise, she might not use Persian with her granddaughter and that would be a shame to lose it
She'll get enough English outside the home so it's not necessary that your dh speak a non-native language to his own child. It's better for the relationship too when she gets older. Experience speaking! Much nicer to explain how an engine works or the birds and the bees...in my native language!!
Don't let the mother-in-law get discouraged and start using English. It's too easy a habit to slide into. So tempting to talk to a child in his or her stronger language. Even my own mom, who doesn't speak French, started saying stuff to my son on a visit
. He taught her! He didn't want to be bothered with English. Get bothered sweetheart and get on the bandwagon mom!!
It's better if each person sticks to his or her language and doesn't mix. I've seen that too many times and the child tends to latch on to the community language, refusing to respond in the parents' language. The minority language can quickly land in the heap. Not too useful if they only understand. The real skill to learn is speaking. Understanding wont go on a resume!
It's okay if the languages are not at the same level, as long as the child responds in the appropriate language and can function in it without having to "dip" into the other. Sometimes my kids have to ask me what something is in English but they can have a whole conversation in it. While their English is not as good as their French, the German lags behind that. The older two can speak it but the little one can only say words and can understand simple sentences.
Two children mixed French and English as toddlers but everywhere I read, it was supposed to be normal and temporary. It was. The middle child, the earliest talker, never mixed. I didn't know she even spoke French till I asked at the day care ("garderie
I've seen plenty of children enter the maternelle
(preschool) without a lick of French and did great by the end of the year. There are a lot of refugee families where we live. I do know of a 4 language family but they were very strict. The kids speak both parents' languages, even though the parents don't speak each others', plus the community and school language (English, what the parents use together and they use as a family).
So it can be done. The only "bummer" about Persian is that while Iran is a beautiful country with a rich and colorful history and culture, the possibility of actually travelling there and visiting is not really in the cards for many years to come. It's very helpful to visit the country of the language, as you will learn with French, but the politics are against your family. Perhaps someday in the future. Where I grew up, we had a lot of Persian families and classmates.
It's so worthwhile to stick to the script. Once the child is older, you can be more flexible. But last spring, my son went to Germany. I listened to him translating between the three languages at 9 years old on the phone. I was so proud. He now wants to be an interpreter when he grows up.
So hope it works out. Certainly, it's realistic and can work.