Customize Mails

I′m using clean login on a multilanguage site using WPML and running on 3 different URL′s.
I would like to customize the opt-in mails send by the system providing the link to confirm registration.

I want to implement a formally salutation which a specific message, using datas from the registration form, for example:

————————–
Dear <first name> <name>,
thank you for sign up at <specific URL containing registration form).
……
————————–

How can I do this, which files do I have to change?

In advance, thank you very much for each help!

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