The study of participation from a socio-interactional perspective relates to the exploration of the interactional practices displayed by interactants and the close examination of the multimodal resources they employ. Here we observe the language choices made by 2 teachers and 10 students who participate in a "WhatsApp" chat over the summer break. Since language choice is a "social category-bound activity" linked to social forms of participation, our analysis focuses on the negotiation participants engage in to determine their "practiced language policy" and to select their "medium of interaction." Our analysis reveals that language choices reshape and construct what we would refer to as an 'in-between learning scenario'. This scenario creates tensions between the teachers' and the students' agenda, observed through their alignment with other interactants and their interpretation of the task at hand. Language selection is not a strict, personal choice. Instead, it is socially situated and depends on how speakers co-construct the communicative event they participate in and who holds the control of topic selection.