There is no better way to help reinforce the second (or third) languages than having monolingual relatives that don’t speak English visiting. My mom, our kids call her “Nine-Nine” visits us once a year, and her english is very limited. So when she is around, our kids are more inclined to speak Mandarin. However, my German and English-speaking husband can’t really say much to my mom. Things get interesting when I am out-of-town. My husband came up with the idea that he can get the kids to translate for him.
One Saturday afternoon, he wanted to take the kids to the park so he asked MM to translate, “please tell Nine-Nine that we are going to the park. Ask her if she’d like to go with us. We will be home in an hour.” MM then looked at Nine Nine and said in Mandarin ” we are going to the park, bye!”
I think that is the best. Bilingual kids don’t translate word-by-word. They comprehend, and paraphrase in the most efficient way possible. Do your kids practice “economy of words “?
Raising Bilingual Children in America 
My three year old recently started praying on her own. And each prayer is identical, and three sentences long. Economical, indeed!