I created a new MySQL database and connected Liferay to it. It created the email@example.com user.
I went to My Account and changed the firstname, lastname, email and screen username to my settings.
I set the default language for the Portal Settings to French, then went into My Account and set the language to English. So all users should see the portal in French while I can use the portal (and especially the Control Panel) in English.
Unfortunately, when I sign-out and then sign-in again, everything is back in French and in My Account I see French as language.