My daughter had the same problem when she went to preschool at the beginning of this year. 我哋平時喺屋企講中文, but when it's story time, we would read the story in English and explained it in Chinese to her. She can pick up some simple English words from the story and applied to her day-to-day activity at school. It's been 9 months now, and she finally enjoys going to school.
i don't think language is the major problem. as u said, separation anxiety might be the problem instead. if your kid kept crying, how was he going to hear what the teacher said?
we also speak Cantonese to our son since he's a baby. he started preschool when he was 2. at that time, he didn't even know any English at all. on the first day of school, he was able to ask the teacher for a band-aid by showing his finger with a cut/scratch. on the next day of school, he already picked up some simple phrases/sentences from some of his classmates.
if i were u, i wouldn't break our family tradition of speaking our own language at home. just be patient to wait for a month or so .... your kid might be different!
i have a friend who was really mad at the teacher and thought she was inpatient when she was told the same thing (speak English to her kid at home). eventually they pulled out their kid from that preschool.
well. we only speak Cantonese at home, Amanda speaks Cantonese and English to Gabriel. our Nanny can't speak English. Gabriel has no problem to go to day care even he has speech delay, just because that is his disability. he is a gifted, and that is disability too because he has tunnel vision.
I agree with SY mom, find out if separation anxiety is the reason :)