You can have only one confirmation mail. Therefore only language constants are used in the confirmation mail. These are automaticly filled with text in the language as needed.
In case you want to change the text you can edit the language files in Acymailing. In Configuration->Languages choose the language you want to change. A new window with two panels opens. In the top panel find the language constant you want to change, copy the entire line and paste it in the bottom panel. There you can change the text string as needed.