Because my organization mainly works with Americans, its website is 100% in English although the members 100% use a Vietnamese. Before we decide to close the comment section in the website, a person in Vietnam1 commented in English and we decided to email him back. I don't mind answering in English, but as a non-native speaker, one will be more comfortable to read in his/her mother language. I think I can build a better relationship by using Vietnamese with a Vietnamese person.
Should I reply to him in Vietnamese?
If I need to ask the customer what does he prefer, what should I ask? Just "I wonder if you want to communicate in English or in Vietnamese?"? Should that sentence be in English or not? And should we include the answer for his question in that email too?
1 His name is apparently a Vietnamese name, and his address is in the country, and I can guarantee that he is a native speaker.