This paper presents a collaborative research in Genevan elementary multicultural and multilingual classrooms to promote classrooms' social climate and inclusion, thanks to a multilingual cooperative program. The program relies on linguistic and cultural diversity awareness and cooperative learning with specific attention to the students' status in line with complex instructions. It includes activities for preparing students to constructively cooperate before integrating multilingual cooperative activities, using students' heritage language. The program was adapted for different levels (grade 3, 4 and 5) and implemented by three elementary teachers in their own class. Multilingual cooperative activities rely on the translation of a text into students' languages (from 11 to 13 different languages per class) in order to create multiple-ability tasks. We summarize parents and students' feedback, as well as the evolution of students' sense of classroom belonging and satisfaction of the need for relatedness.