This is inspired by Victoria's comment on a previous post.
Students should be encouraged to use English not only to do a task but also to interact with one another while they are doing the task. This is a real opportunity for the students to use the language for a real purpose. We mustn’t throw it away.
"Social skills such as acknowledging another's contribution, asking others to contribute, and keeping the conversation calm need to be explicitly taught." (Section on Cooperative Learning)
Diane Larsen-Freeman's Techniques and Principles in Language Teaching
If you are teaching beginners, the interaction has not to be complicated. You can limit it to some very basic social skills. You can actually start by doing a pre-group work activity. Just bear in mind that learning to work in groups usually stretches over a quite long period. Therefore, you can teach your students one or two things at a time.
For instance, you can start by teaching them how to ask for their turn if they are playing a language game or doing something together. Think of unchallenging phrases like:" Now, it is my turn", "is it your turn?", "who is next?",etc…
You can start by asking how your students say it in their language. Then you provide the equivalent in English. You can write the expressions on the board and have your students repeat after you.
Next, you can teach them how to encourage each other during an activity or when they finish it. Think of expressions like "Nice Job!", "Excellent", "that's good", "Well done", "Perfect!" etc...
Step 3: sitting in groups
After that, you can ask your students to sit in groups and give them flash cards with the expressions in step1. Some of the flash cards will have the words in English written on them. The others will have the words in the students' native language or pictures/illustrations, etc...
Explain that the students have to match the phrases in English with those in their language. Make it clear that they should encourage each other using words from step two.
To encourage this behavior and make them refrain from using their native language, you may tell your students they will only get points if they use English.
Keep a checklist/ form (grading/points) and go from one group to another to make sure they do use English.
Remember that you can model this by encouraging your students in English when you go from one group to the other. By habitually doing this, your students are more predisposed to learn it.