For English as a Foreign Language (EFL) learners, code-switching (CSW), or alternating between their native language and the target language (English), can lower anxiety and ease communication barriers. Large language models (LLMs), with their multilingual abilities, offer new opportunities to support CSW in speaking practice. Yet, the pedagogical design of LLM-based tutors remains underexplored. To this end, we conducted a six-week study of LLM-mediated speaking practice with 20 Korean EFL learners, alongside a qualitative study with nine English teachers who designed and refined responses to learner CSW. Findings show that learners used CSW not only to bridge lexical gaps but also to express cultural and emotional nuance, prompting teachers to employ selective interventions and dynamic scaffolding strategies. We conclude with design implications for bilingual LLM-powered tutors that leverage teachers' expertise to transform CSW into meaningful learning opportunities.

翻译：对于英语作为外语（EFL）学习者而言，语码转换（CSW），即在母语与目标语言（英语）之间交替使用，能够降低焦虑感并缓解沟通障碍。具备多语言能力的大型语言模型（LLMs）为支持口语练习中的语码转换提供了新的机遇。然而，基于LLM的辅导工具在教学设计方面仍待深入探索。为此，我们开展了一项为期六周的研究，观察20名韩国EFL学习者在LLM介导下的口语练习，并同时对九名英语教师进行了质性研究，这些教师设计并优化了对学习者语码转换的回应策略。研究发现，学习者不仅通过语码转换弥补词汇空缺，还借此表达文化与情感层面的细微差异，这促使教师采用选择性干预和动态支架策略。最后，我们提出了双语LLM辅助教学工具的设计启示，旨在借鉴教师专业经验，将语码转换转化为有意义的学习契机。