This study examined cultural content in an EFL textbook, Bahasa Inggris: Work in Progress, and explored English teacher’s localization practices to fit students’ needs. The data were collected through document analysis and interviews with one high school English teacher. The findings revealed that the textbook heavily emphasizes cultural products, while practices, people, and perspectives appear less frequently. Most cultural content focuses on universal and source cultures, with limited representation of target and international cultures. This study also found that the teacher actively localized the textbook by adjusting cultural, linguistic, and pedagogical elements to bridge gaps between the textbook and students’ backgrounds. Grounded in Vygotsky’s sociocultural theory, this highlights the teacher’s role as a cultural mediator who provides scaffolding to support students’ learning within their Zone of Proximal Development (ZPD). It is suggested that textbook developers balance cultural dimensions based on students’ level. Future researchers are recommended to investigate more varied learning materials and educational settings to better understand cultural content and localization in a broader context.