URI for this namespace is http://xml.zope.org/namespaces/i18n. To use i18n: attribute prefix in XML you're required to declare it:
<html xmlns:i18n="http://xml.zope.org/namespaces/i18n" …>PHPTAL does not enforce this requirement.
i18n is a short name for 'internationalization'. This namespace allow template designers to specify some text zones that must be translated during template evaluation.