Say your language is Chinese, of course, local Chinese customers would feel more comfortable communicating in English. The disconnect happens when head office needs to get involved. They can't review emails, read the history of email threads etc.
So make it easier to add English speakers into the loop at a moments notice.
The most polite way of doing this is to mention in the email why you are using English.
Hi Joe, hope all is well. I wanted to confirm our conversation...Figure: Bad example
(This is in English since I have included a Sydney colleague... let me know if you need it in Chinese).Figure: Good example
Hi Joe, hope all is well. I wanted to confirm our conversation...