In college, I found elective classes of my wife and in-laws foreign language. We primarily speak English in our home. My children are not proficient in a foreign language, but over time are accustomed to hearing certain words and phrases.
It is Rare that I ever speak to them in a language other than English, even though their mother and grandparents do.
In Emergency type situations, I have spoken to the children in a foreign language to illicit the response from the children I am looking for.