怎么样委婉的告诉客人使用英语？

您可以尝试使用以下回复，委婉地请求客户使用英语：

Dear [客户名],

Thank you for the design you sent me. In order for me to confirm the details with you accurately, I would like to kindly request that we communicate in English to avoid any misunderstandings. Please let me know if this is possible for you.

Thank you for your understanding.

Best regards,
[您的名字]

这样既表达了您需要使用英语确认细节的需求，也在客气委婉的同时表达了对客户的感谢和理解。希望这个建议对您有帮助。