You know the problem, right? Your learners lapse back into their own language whenever you ask them to do pair- or groupwork, rather than the English they're supposed to be learning.
Here's a brilliant idea that Adriana left in a comment on a previous post on this same issue. Adriana suggests using a clothespeg and explains:
The first student to slip into L1 gets the clothes peg on
their clothes. Their only way of getting rid of the peg is to catch another student speaking in their L1, in which case they can stick the clothes peg on them!
We got some hilarious reactions with that — and it did the job amazingly well. You might want to do it with a class there is already a positive atmosphere in: you don't want to humiliate anyone!
What language were you speaking
Here's a second idea that works — though it's more an awareness-raising thing than anything else.
In class where you have this problem, hand out slips of paper just before starting an activity that should involve the learners speaking a lot of English. Let them get on with the activity, but stop them in the middle of it when you sense that the language being most spoken is not English.
At that point, ask them to write down what language they were speaking when you stopped them. Once they've done that, get someone to come out to the front and tally the results on the board.
I like to draw attention to the results but, while I certainly would encourage discussion of it, wouldn't make a lot of fuss about it: it's awareness-raising, as I say, not a line-up for punishment detail.