We did it slightly differently. I spoke English, told mother to speak only Thai, but she tended to use only English. Though the daughter went to Thai nursery school so picked up some Thai.
At six enrolled in a French international school, so all lessons in French except English obviously. Also have a Thai teacher comes to house for two hours/week.
Now speaks both English and French like a native, and just started Spanish. . Can speak Thai, but definitely needs more work.