This study investigated the L2 motivation of five Chinese undergraduates learning Bulgarian as their major and studying in Bulgaria in a one-academic-year exchange programme. The focus of the study concerned a rarely explored L2 learning situation, namely when the L2 is associated with a host community of limited ethnolinguistic vitality. Based on the theoretical foundations of the L2 Motivational Self System and the ideal multilingual self, and drawing on the principles of retrodictive qualitative modelling, this study identified three distinct motivational patterns emerging during the one-year study abroad in Bulgaria: decreasing motivation with a weakened ideal Bulgarian self; fluctuating motivation with a weakened ideal Bulgarian self; and fluctuating motivation with a strengthened ideal Bulgarian self. A comparative analysis of these patterns suggested that: (1) language learners' agentic interpretations of their sojourn in the host country had a considerable impact on their cultural interest, thereby shaping the quality of their ideal Bulgarian self; (2) the impairment/enhancement of the ideal Bulgarian self was a principal cause of the decrease/increase of the participants' Bulgarian-learning motivation; and (3) an ideal multilingual self can work as compensation for the impaired ideal Bulgarian self, by directly motivating student's learning when the ideal Bulgarian self was weakened.