This paper investigated the effect of a Collaborative Online International Learning (COIL) project on English as a Foreign Language (EFL) learners' self-perceived communication competence (SPCC), communication anxiety (CA), and soft skills. The participants were 157 university students from Japan, Türkiye, and Spain who completed various tasks and created a video on Sustainable Development Goals during the fall of 2022. Students answered two surveys (pre-COIL and post-COIL) through a mixed-method convergent research design. Both surveys included three different 5-point Likert scales (analyzed via t-test and convergence analysis), and the post-COIL survey included additional open-ended questions for qualitative analysis. The t-test analysis demonstrated a statistically significant increase in EFL students' average SPCC. The convergence analysis showed that students with the lowest and highest SPCC, CA, and soft skills converged towards the average and experienced the greatest change in these traits post-COIL. Considering the qualitative data and social cognitive theory, participation in the COIL assisted students in better understanding their SPCC, CA, and soft skills when working in an international group.