First of all, you cannot be 100% sure the members 100% use a non-English mother language. You always have the 0.00...01% chance that there is some member who understand only English.
Secondly, you always want to do your best to honor the customers' requests if you can.
Having said that, my answer to your question is: use English for the first e-mail reply to the customer. In that reply, you can say "I wonder if you want to communicate in English or in [mother language]?" in English.
If the customer's reply says, "Yes, I would like to use [mother language].", then you can start to use that [mother language]. Otherwise, continue to use English.
Lastly, let me give you my personal experience. I use both English and Chinese. But, there is no Chinese input software on my main computer because I did not install it. Inputting Chinese on that computer will be a pain for me. So, if I were one of your customers and you send me email using Chinese assuming I use Chinese, I probably will walk away and become your non-customer.
The main spirit of my answer is: don't assume anything until you get confirmation from your customer.
