In recent years, researchers have become increasingly interested in mobile-assisted language learning (MALL) studies for online informal learning of English as a foreign language (EFL). Smartphones, and other mobile devices, have been used as a powerful tool in informal contexts of learning since that form of technology can help students engage in out-of-class language learning experiences. However, to our knowledge, no research on EFL learners' predominant language use for their online informal learning activities has been conducted in Indonesian context. Therefore, this study attempted to investigate EFL learners' predominant language use for their online informal learning of English through smartphones, in which the data collected were quantitatively analyzed through descriptive statistics. The results indicate that EFL learners predominantly perform their online informal learning activities in Indonesian, in English, and in both Indonesian and English. Finally, suggestions and recommendations for future research are also discussed.