How to validate drop down list based on checked checkbox

I need your help in validating a drop down list based on the checked checkbox. Currently, I am having many checkbox item which has list of certificates

<h:selectManyCheckbox id="certificates"
                                layout="pageDirection" required="true" requiredMessage="Error: Select at least One Certificate">
            <f:selectItem itemLabel="Certificate A"
                          itemValue="c_a"/>
            <f:selectItem itemLabel="Certificate B"
                          itemValue="c_b"/>
</h:selectManyCheckbox>

My scenario is that if the user selects Certificate A, then the drop down list language should be enabled and it should be required.

            <h:selectOneMenu label="language" disabled="true">
              <f:selectItem itemLabel="Please Select a Language"
                            itemValue=""/>
              <f:selectItem itemLabel="A" itemValue="A"/>
              <f:selectItem itemLabel="B" itemValue="B"/>
            </h:selectOneMenu>

So how can I achieve this and Thanks