Fixed 'Undefined index: id' on translation save.

This warning is produced when calling Link->add() because the Link
model doesn’t have a single-field primary key. Instead we use a
composite primary key. However (quoting Cakebook [1]):

> All tables with which CakePHP models interact (with the exception of
> join tables), require a singular primary key to uniquely identify
> each row. If you wish to model a table which does not have a
> single-field primary key, CakePHP’s convention is that a single-field
> primary key is added to the table. You have to add a single-field
> primary key if you want to use that table’s model.

[1] http://book.cakephp.org/1.3/en/The-Manual/Basic-Principles-of-CakePHP/CakePHP-Conventions.html