L2 English and L3 Japanese motivation, international posture, and success of students in an English-medium instruction (EMI) program at a Japanese University

English-medium instruction (EMI) is growing in popularity worldwide, including in Japan. However, students' language learning motivation has not been fully researched in this context. Accordingly, this mixed-method study explores the roles of L2 English and L3 Japanese motivation in an EMI programme at a Japanese university. Applying L2 motivational self system and International Posture (IP) as theoretical frameworks, quantitative data from surveys (n = 66) and qualitative data from eight semi-structured interviews with international students were collected at an EMI programme of Policy Studies in Japan. The results indicate that motivation to learn L2, especially ideal L2 self, plays a significant role in student studies. Furthermore, a strong IP, including viewing study abroad (SA) in Japan as 'an international learning environment' where they can 'experience the world', was related to their use of English. In contrast, L3 motivation decreased for most participants owing to the lack of integrativeness, scarcity of opportunities to use L3, and negative learning experiences. In other words, SA in Japan does not automatically motivate students to learn Japanese.