The number of migrant workers in South Korea is on the rise, but their inadequate Korean language skills prevent them from being promoted at work, or fairly treated as respected members of the society. In this study, in collaboration with a government-authorized language educational facility for immigrants, the authors investigated (a) challenges in migrant workers’ Korean as a second language learning, and (b) design principles of lessons and learning materials specifically targeted to their needs. Student and teacher interview data confirmed that the workers’ limited time for study, weak motivation, Korean colleagues’ indifferent attitude, and limited teaching resources at educational facilities are major barriers to achieving higher levels of linguistic skills. From the data, the authors identified four design principles: personalized content, community participation, portability of materials, and micro learning modules. Informal lessons via Facebook, factory safety signs, and portable writing drill booklets are designed as on-going experimentations of the principles.