Why the template_preprocess_field hook is not called for a custom template?
As @Clive says the problem was that the code didn't use the render() function for the field output, and in this case the template_preprocess_field() hook is not called.

For the moment I have 2 solutions:


Use the render() function in the template file (node--emploi.tpl.php) to create the output of the field. In this case the template_preprocess_field() hook is called.
Use the template_preprocess_node() hook and check $vars['type'] for my content type. In this case the template file remains equal.