Zend下拉菜单标题不会被翻译

Question

I have a field county:

    $county = new Select('county');
    $county->setAttribute('id', 'county');
    $county->setAttribute('class', 'stored');
    $county->setAttribute('title', _('web.form.selfService.register.county.title'));
    $county->setAttribute('required', 'required');
    $county->setEmptyOption(_('web.form.selfService.register.county.emptyOption'));
    $county->setValueOptions($this->getDomainValues(Domain::COUNTY));
    $county->setLabel(_('web.form.selfService.register.county.label'));
    $this->add($county);

When the county field is left empty and form is submitted the output is the source text: web.form.selfService.register.county.title

Also I have many text fields in the form and their titles are translated fine.
Example:

    $firstName = new Text('firstName');
    $firstName->setAttribute('id', 'firstName');
    $firstName->setAttribute('class', 'stored');
    $firstName->setAttribute('placeholder', _('web.form.selfService.register.firstName.placeholder'));
    $firstName->setAttribute('title', _('web.form.selfService.register.firstName.title'));
    $firstName->setAttribute('required', 'required');
    $firstName->setLabel(_('web.form.selfService.register.firstName.label'));
    $this->add($firstName);

and when first name field is left empty and the form is submitted the output will be:
Please insert first name