I really doubt you have anything to worry about. I've seen with a neighbor boy, brought up with both German and English spoken in the home, and with another Japanese and English household, that sometimes kids are very resistant to speaking out loud until they've got both languages fully sorted out in their heads. Then one day they just start talking fluently as if there was never a problem.
Pushing them to "perform" before then is probably counterproductive. Your anxieties may actually be internalized by your children as being caused by some deficit your perceive in them, or some expectation of excellence, which might make them all the more anxious about appearing stupid. Especially when you turn a spotlight on them in front of other adults and demand that they speak. That can make a child terribly uncomfortable (how do you spell "squirm?").
I wouldn't worry about other adults judging them as impolite or stupid. These are young children, recently moved from another culture. Adults will cut them some slack.
I hope you'll cut them some slack, too. Ignore their choice to stay mum, other than to tell them you are confident that they'll speak when they know they are ready. If I were in your shoes, I might even apologize for pushing so hard. Trying to "break" them is an extremely unhelpful way to look at any emotional dynamic in human beings, child or adult.