The source node in most cases contains the sample message as written by the developer. If it looks like a dot-delimitted string such as "form.label.firstname", then the developer has not provided a default message.

Hello, Join us at: %invite_link% Witaj, Dołącz do nas w: %invite_link% key: ibexa.user.invitation.mail.message You are invited to join Jesteś zaproszony aby dołączyć key: ibexa.user.invitation.mail.subject Invitation for '%email%' already exists. Zaproszenie dla '%email%' już istnieje. key: user_invitation.send.invitation_exist Invitation sent to '%email%' updated. Zaproszenie wysłane do '%email%' zaktualizowane. key: user_invitation.send.success User with '%email%' already exists. Użytkownik z tym adresem e-mail już istnieje key: user_invitation.send.user_exist