<?php
class Kwc_FulltextSearch_Search_ViewAjax_SearchForm_FrontendForm extends Kwf_Form
{
    protected function _initFields()
    {
        parent::_initFields();
        $this->_model = new Kwf_Model_FnF();
        $this->fields->add(new Kwf_Form_Field_TextField('query', 'Query'))
            ->setAutoComplete(false)
            ->setHideLabel(true)
            ->setClearOnFocus(true)
            ->setDefaultValue(trlKwfStatic('Search'));
    }
}

<?php
class Kwc_FulltextSearch_Search_ViewAjax_SearchForm_FrontendForm extends Kwf_Form
{
    protected function _initFields()
    {
        parent::_initFields();
        $this->_model = new Kwf_Model_FnF();
        $this->fields->add(new Kwf_Form_Field_TextField('query', 'Query'))
            ->setAutoComplete(false)
            ->setHideLabel(true)
            ->setEmptyText(trlStatic('Suchbegriff'));
    }
}

Use setEmptyText instead of default-value and clearOnFocus
Use setEmptyText instead of default-value and clearOnFocus
