Research has found that teaching about diversity, equity, and inclusion (DEI) can raise U.S. high school students' awareness about DEI-related topics and develop their critical thinking skills to analyze DEI-related issues. But the achievements of DEI-focused education are limited. Thus, DEI-focused education needs complementary efforts to enhance student outcomes. Research has also found that learning an additional language (LX) may heighten students' interest in learning about other cultural communities, nurture students' cross-cultural empathy, and elicit culturally responsive behaviors. The juxtaposition of these findings signifies the potential for LX learning to complement DEI-focused education to improve learning outcomes. However, the value of LX learning for DEI goals has not received much attention in the literature. This mixed methods phenomenological study aims to explore the relationship between instructed non-immersion LX learning and student outcomes of DEI-focused education in cognitive, affective, and behavioral aspects among U.S. high school students. In total, 76 U.S. high school students participated in this study. The research questions addressed by this study include: (1) What are the students' perceptions of the achievements and limitations of DEI-focused education? (2) To what degree are the length of the students' LX learning in an instructed non-immersion setting, the frequency of guided reflection on cross-cultural differences in instructed non-immersion LX classes, and the percentage of course time spent on such reflection associated with their intercultural competence? (3) Based on the students' perceptions, what are the possible influences of instructed non-immersion LX learning on DEI-focused education in cognitive, affective, and behavioral aspects? This study has found that students acknowledge the achievements and limitations of DEI-focused education and see instructed non-immersion LX learning as a complementary experience to DEI-focused education. None of the LX learning factors examined by the second research question can reliably predict students' appreciation of DEI. To actualize the complementary function, LX teachers need to explicitly draw connections with DEI-related topics and purposefully address certain factors in their LX curriculum design and teaching practice. Besides implications for teaching, these findings may also deepen people's appreciation of the developmental benefits of LX learning and encourage policies to support LX programs. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]