Collaborative writing (CW) involves two or more students writing a single text together. Previous studies mainly focused on students' cognitive engagement in CW and investigated their attention to various language-related problems during task interaction. However, little CW research to date has considered that engagement in language-related discussions can manifest from three dimensions: cognitive, social, and affective. Focusing on the multidimensional characteristics of engagement, this study investigated how Russian learners' social and affective reactions influence their focus on language use while they completed a CW task. Drawing on Svalberg's framework of engagement with language to identify the three dimensions of engagement, I conducted a mixed-method approach towards analysing the audio-recorded collaborative dialogues by three student pairs (n = 6), along with a qualitative analysis of their responses to a five-point Likert scale questionnaire. The analyses showed that when learners were interactive and viewed the activity as useful, they noticed many linguistic problems and elaborated on them. In contrast, when learners demonstrated social disengagement and perceived disadvantages from CW, they were likely to withdraw their attention from resolving the language issues they encountered. These findings indicate the complex and dynamic nature of task engagement. They can provide second language (L2) teachers with an in-depth understanding of how to fully engage students in instructional activities to better foster their L2 learning.