The various System Pages are available from the drop down List as below, and are all Editable to suit your application, which obviously accommodates any specific language requirements. The System pages cover various functions, including what go out as part of a Form (if you wish) and in response to a new subscriber (if you wish).
There is a Subscription Confirmed Message that can be displayed immediately someone Subscribes. However, most Users would prefer to send the new Subscriber to a Welcome / Thankyou page via a Re-Direct URL
That Re-Direct URL is set here as below via Dashboard > Lists > Your List > Overview > Pages > Dropdown list top right hand corner > Subscription Confirmed > Custom Assets > Re-Direct URL
Set the Re-Direct URL as shown below. You can of course send a Welcome/Thankyou Autoresponder to a new subsciber as well.
Check out those System Pages that are available from the Dropdown List top right, and all are Editable, including the Double Optin Confirmation Email.