There should be no reason why an ESL
teacher should have to use Chinese to teach English

I assume you are referring to a FT teaching Oral English. In an EFL environment, use of the native language is critical in teaching English grammar and writing. On the university level all students, including English majors, receive instruction from Chinese English instructors.

My grammar knowledge is excellent and my Mandarin oral language ability is intermediate. I have explained to students, in both English and Mandarin, the use of modality in 'like' and 'love' in their use of gerunds and infinitives. But I am confident an Chinese English professor does a better job in contrastive instruction in Mandarin, which is precisely what the students need. Imagine studying Chinese grammar completely in Chinese from a person who does not speak English.

In an actual ESL environment, say learning English in London, there is much debate about the use of the '1st' language in the classroom. During my time at Columbia during the last century, it was dismissed as counter productive for teaching adults. I'm not up to date on language acquisition pedagogy.