As far as I know,
You will have to make entry of alfresco-global.proeprties file in your custom-sligshot-application-context.xml file.
<beans>
<bean id="webscripts.resources" class="org.alfresco.i18n.ResourceBundleBootstrapComponent">
<property name="resourceBundles">
<list>
<value>alfresco-global</value>
</list>
</property>
</bean>
</beans>
After this, you can access property in presentation webscript ftl file like, ${msg("dir.root")}.
Hope this helps.