[shishapipe]

I have added to the SQL database the below code and it seems to work ok, just need to sort out the emails. The above code works ok with a login user or a guest user now after adding the following SQL files.
I added these manualy in the Language section for text, email and Lables.
You just need to do one language and it replicates for the other 3 languages. (SE, FR and DE)

INSERT INTO xcart_languages VALUES ('DE', 'Notify Me', 'txt_addme_notify', 'Please enter your email in the box provided and we will notify you when stock arrives in.', 'Text');
INSERT INTO xcart_languages VALUES ('FR', 'Notify Me', 'txt_addme_notify', 'Please enter your email in the box provided and we will notify you when stock arrives in.', 'Text');
INSERT INTO xcart_languages VALUES ('SE', 'Notify Me', 'txt_addme_notify', 'Please enter your email in the box provided and we will notify you when stock arrives in.', 'Text');
INSERT INTO xcart_languages VALUES ('US', 'Notify Me', 'txt_addme_notify', 'Please enter your email in the box provided and we will notify you when stock arrives in.', 'Text');
INSERT INTO xcart_languages VALUES ('US', 'Stock Notify', 'txt_added_stock_notify', 'Thanks you will be sent a email when stock arrives back.', 'Text');
INSERT INTO xcart_languages VALUES ('SE', 'Stock Notify', 'txt_added_stock_notify', 'Thanks you will be sent a email when stock arrives back.', 'Text');
INSERT INTO xcart_languages VALUES ('FR', 'Stock Notify', 'txt_added_stock_notify', 'Thanks you will be sent a email when stock arrives back.', 'Text');
INSERT INTO xcart_languages VALUES ('DE', 'Stock Notify', 'txt_added_stock_notify', 'Thanks you will be sent a email when stock arrives back.', 'Text');
INSERT INTO xcart_languages VALUES ('DE', 'Stock Notify', 'lbl_stock_notify', 'Stock Notify', 'Labels');
INSERT INTO xcart_languages VALUES ('FR', 'Stock Notify', 'lbl_stock_notify', 'Stock Notify', 'Labels');
INSERT INTO xcart_languages VALUES ('SE', 'Stock Notify', 'lbl_stock_notify', 'Stock Notify', 'Labels');
INSERT INTO xcart_languages VALUES ('US', 'Stock Notify', 'lbl_stock_notify', 'Stock Notify', 'Labels');
INSERT INTO xcart_languages VALUES ('DE', 'Keep me updated', 'lbl_stock_notify_button', 'Keep me updated', 'Labels');
INSERT INTO xcart_languages VALUES ('FR', 'Keep me updated', 'lbl_stock_notify_button', 'Keep me updated', 'Labels');
INSERT INTO xcart_languages VALUES ('SE', 'Keep me updated', 'lbl_stock_notify_button', 'Keep me updated', 'Labels');
INSERT INTO xcart_languages VALUES ('US', 'Keep me updated', 'lbl_stock_notify_button', 'Keep me updated', 'Labels');
INSERT INTO xcart_languages VALUES ('DE', 'The following product is now back in stock and can be ordered by clicking on the link below', 'eml_stock_notification', 'The following product is now back in stock and can be ordered by clicking on the link below', 'E-Mail');
INSERT INTO xcart_languages VALUES ('FR', 'The following product is now back in stock and can be ordered by clicking on the link below', 'eml_stock_notification', 'The following product is now back in stock and can be ordered by clicking on the link below', 'E-Mail');
INSERT INTO xcart_languages VALUES ('SE', 'The following product is now back in stock and can be ordered by clicking on the link below', 'eml_stock_notification', 'The following product is now back in stock and can be ordered by clicking on the link below', 'E-Mail');
INSERT INTO xcart_languages VALUES ('US', 'The following product is now back in stock and can be ordered by clicking on the link below', 'eml_stock_notification', 'The following product is now back in stock and can be ordered by clicking on the link below', 'E-Mail');

Also added to SQL the following:

INSERT INTO xcart_modules VALUES (2,'stock_notify','Allows registered users to ask to be notified when a product is back in stock','Y');

CREATE TABLE xcart_notify (
  email char(128) NOT NULL ,
  productid int(11) NOT NULL ,
  KEY email_proudct (email, productid),
) TYPE=MyISAM;