In self-determination theory (SDT), the role of positive emotion in the motivational process is undertheorized. Most previous research considered emotion as an outcome variable, rather than a main construct of the motivational process. There is, however, some evidence in domains outside language learning suggesting that positive emotion moderates the motivation-engagement link. The extent to which this could be applied to the domain of language learning remains unknown. Therefore, this study aimed to examine the role of positive emotion as a moderator of the motivational process of language learning. Moreover, a comparative multilingual perspective was adopted to better understand simultaneous language learning. Specifically, this study assessed whether positive emotion moderated the relationship between two types of motivation (i.e., autonomous and controlled motivation) and behavioral engagement in two foreign languages. A sample of 108 Chinese university students concurrently learning English as L2 and French as L3 participated in the survey study. Results of regression analysis suggest the differential role of positive emotion and motivation in English and French: For French learning, positive emotion served as a moderator that optimized the motivation-engagement link (i.e., positive interaction between emotion and motivation). For English learning, no moderating effect was discovered, with positive emotion being the significant and strong predictor of engagement. The findings supplied initial evidence of the potential role of positive emotion as a moderator of the motivation-engagement link, contributing to the theoretical incorporation of emotion as a more central player in the motivational process proposed by SDT. The differential role of positive emotion and motivation in learning English and languages other than English points to the value of a multilingual comparative perspective to uncovering the language-specific characteristics of the motivational processes.