I heard a dad who would repeat every single thing the child said in the "wrong language". That is translate it and say it back to them before answering. It makes them learn but nearly more importantly it bores them out so much to just hear what they say repeated in another language they get motivated to speak the "correct" one to you. It works kinda like this:

Kid: I want to go on the trampoline with my friends tomorrow (in the "wrong" language)

Patent: Ah you want to go on the trampoline with your friends tomorrow? Well of course you can, just be careful (in the "right" language)