You can change the language of the Webmail interface by clicking on "Einstellungen" then "Sprache, Zeit, Datum" and then chose "English" from the dropdown list ("Sprache").
This obviously doesn't change the language of the mails you receive 😉
(If you'd like top receive service mails from Swisscom in English, you'd probably have to change your "Korrespondenzsprache". However I'm not sure if you can do this by yourself or if you have to ask the hotline.)