Reading engagement is a strong predictor of students' reading outcomes, but its consistent positive effects across diverse student groups remain unclear. Research on the reading engagement of multilingual adolescents is notably limited. We investigated the interactions of affective and cognitive dimensions of reading engagement in relation to reading achievement among multilingual and English-dominant students. Specifically, we explored how reading motivation is related to reading achievement through metacognitive strategies. For a nuanced understanding of reading engagement, we further examined whether these relationships are distinct for students with different language backgrounds. We conducted multi-group structural equation modeling using data from the U.S. Programme for International Student Assessment 2018. Our analyses included 2,928 students: 2,407 English-dominant, 359 Spanish-speaking, and 162 other-language-speaking multilingual students. We found differential relationships between reading engagement and reading achievement across language groups. For English-dominant and Spanish-speaking students, reading motivation had both a direct and indirect effect on reading achievement through metacognitive strategies. In contrast, for other-language-speaking students, motivation was only linked to achievement through metacognitive strategies, with no direct contribution from motivation. Our results suggest that metacognitive reading strategies were a critical explanatory mechanism for translating reading motivation into reading achievement. For effective reading instruction, integrated instructional practices that support both metacognitive strategies and motivation are necessary, with a tailored approach that adapts responsibly to linguistic differences among students.