As globalization and migration intensify, heritage language attrition has emerged as a critical concern, particularly among second- and third-generation immigrant communities. Despite early exposure, many heritage speakers lose proficiency due to limited use and societal pressure to assimilate linguistically. This study investigates the state of Kazakh heritage language use and evaluates the effectiveness of the Kazakh for Heritage Speakers Course, a culturally grounded educational intervention designed for ethnic Kazakhs residing in the Russian Federation. Utilizing a quasi-experimental, single-group pretest-posttest design and qualitative interviews, the research examined three dimensions: (1) patterns of Kazakh language use among heritage speakers, (2) changes in language proficiency after course participation, and (3) participants' perceptions of the course. A total of 100 ethnic Kazakhs living in Russia participated in the study. Heritage language use was found to be infrequent in daily life, mostly limited to interactions with older family members. Despite this, learners expressed strong intrinsic motivation to reconnect with their linguistic and cultural roots. Pre- and post-course assessments using the Language Assessment Scale revealed a statistically significant improvement in Kazakh language proficiency (p < 0.001), supporting the effectiveness of the intervention. Interview feedback indicated high levels of satisfaction with the course's structure, accessibility, and instructional design. Participants highlighted the value of native-speaking mentors, multimodal materials, and progressive learning tasks. Suggestions for improvement included more conversational practice, extended course duration, and the inclusion of cultural content such as songs and poetry. This study contributes to addressing practical and empirical gaps in heritage language education by offering a scalable, evidence-based model for revitalizing underrepresented languages in diasporic contexts. Findings offer implications for language policy, curriculum development, and future research in heritage language pedagogy.