<ww:textfield label="%{getText('i18n.label')}" name="label1" />

This will get the localized text message for the key "i18n.key" and put it in the label.

Alternatively, portion of controlheader-core.ftl in /template/xhtml could be modified (if xhtml theme is being used), as follows :-

${parameters.label?html}:<#t/>

<#assign mm="getText('"+parameters.label?html+"')" /><#t/>
${stack.findValue(mm)}:<#t/>

or

${stack.findValue("getText('"+parameters.label?html+"')")}

such that using text tag like following will work as well (such that the label printed will be i18n).

<ww:textfield label="i18n.label" name="label1" />