Add translation to in_db validation

Author: kennylajara

system/Language/en/Validation.php

@@ -35,6 +35,7 @@
    'greater_than'          => 'The {field} field must contain a number greater than {param}.',
    'greater_than_equal_to' => 'The {field} field must contain a number greater than or equal to {param}.',
    'in_list'               => 'The {field} field must be one of: {param}.',
+   'in_db'                 => 'The {field} field must contain a previously existing value in the database.',
    'integer'               => 'The {field} field must contain an integer.',
    'is_natural'            => 'The {field} field must only contain digits.',
    'is_natural_no_zero'    => 'The {field} field must only contain digits and must be greater than zero.',