E-mail notification problem

I translated the E-mail content to local language. See example below.
ORDER_STATUS_NOTIFICATION_SUBJECT="รายการสั่งซื้อเลขที่ %s ในขณะนี้ได้ถูก %s ที่ %s"
ORDER_CREATION_NOTIFICATION_SUBJECT="รายการสั่งซื้อเลขที่ %s ในขณะนี้ได้ถูก %s ที่ %s "

One thing that I found out. After customer has placed an order, system will send out e-mail notification to customer. It was working ok for me. The content is shown in my local language. However, when I went to the Control Panel to manage order, I changed the status to, forexample, "Confirmed" and I selected the option to send notification to customer. The system sent out the e-mail notication in English. Very weird. I think I might not set something correctly. Can you please advise? Thanks.

The system doesn't know the language of the user who made an order. If you use your website back end in english, the notification will be loaded in english. So you should use your back end in your own language and the system will load the notification in your own language and not in english.

I tried to set at the Joomla menu. Extensions->Language Manager->Administrator
I set the default to other language. I tested it again but the notification still came out in English. Did I set at the right mene?

When you go to your back end. You have the login interface with three fields: username, login and language. There, select the appropriate language and login.

It works now. Thanks.