replace the word VAT by TAX

Hi,
When someone ordered from the site I got a confirmation of the order to my email.(Please see attached)
I want to replace the word VAT in the sum line with TAX .I know that should be in the language but don't want to mess up something just tell me how and where to modify?
Thank you so much.

Nicolas,

You don't copy or paste or highlight or change anything in the box above only put the one you provide in the box below?that's it?

(Please see attached)

I got it so you copy TOTAL_WITHOUT_VAT="TOTAL without VAT : %s"
from the file above to the box below and change the word VAT to TAX and save right?