RL, had to get off my lazy butt and dig out the files as my memory of the details is fuzzy. I doubt that they speak Thai, as they are in Manila. But no problem, as you are both on the phone at the same time so just put it on speaker phone and you can translate or explain. Mainly they just go over the details, verify names and addresses, etc. Wife also has to swear that the child has not been adopted, that she understands that she is not a US citizen and does not have 5 years of US residency and is not eligible for benefits for herself. They emphasized this point, so I guess there is a lot of misunderstanding about this. And of course the usual stern warning that if any of the information we gave is false we could go to jail.
You said the child already had a SSN card, that's a change. When I applied for her passport they just gave me the application for the card, along with certified copies of everything we would need and told me to send it to Manila. I sent it along with everything else and when Manila got it they called and scheduled the phone application. Here's what I sent to Manila:
1. Form SS-5 Application for Social Security card
2. Thai passport for mother *
3. Certified copy of Baby's American Passport
4. Certified copy of Consular Report of Birth abroad
5. Certified copy of Baby’s Birth certificate
6. Certified copy of Father’s passport
* They only wanted a copy of wife's Thai passport certified by the US Embassy, but they neglected to do that when we were at the Embassy, so Manila said just send the passport and they would send it back, which they did (at their expense!).
I am not sure about the retroactive aspect. Our first check was retroactive for two months since we applied when she was one month old. I've been told that the maximum is six months by another expat, but check this with Manila.
By the way, we applied at the US Consul in Chiang Mai, as I avoid Bangkok at all costs. The Consul told me that I could have applied at any US Embassy in the world, like Laos. They were very accommodating. In fact we applied for her passport on a Thursday afternoon and the following Tuesday I got an email from the consulate saying the passport was ready for pickup! I thought they had made a mistake so I called the consul and made him go get the passport and verify that it was hers. I don't know how they pulled that off, unless they sent it to the State Department by diplomatic pouch.
One caveat in all this, we applied over two years ago and things may have changed, so by all means call Manila and find out exactly what the procedure is nowadays.
Good Luck and congratulations to our newest American citizen.