First I click the 'notify customer' and add order history at Orders > Status & Comments.
Then I changed the language definition using the text I found from the email below:
Order ID: 20
Date Ordered: 28/12/2013
Your order has been updated to the following status:
Processing
To view your order click on the link below:..
The similar text block was 'mail_order_update' but no change occurred when I edited the translation field.
Fyi, I only know html and css so I'm not sure what to do with the link you provided earlier.