Consider the following Wicket markup:
This is only sticking the i18n string (label.toolbar.gradeitemshowall) in the 'value' attribute of the span tag. The text that is actually displayed is the hardcoded value of 'Show All'. If you changed the corresponding message bundle value, the text displayed will remain as 'Show All', and only the text in the 'value' attribute will be changed accordingly.
A more appropriate way to do this is to use a <wicket:message> element for the actual text content: