plone4: how to override translations in plone.app.locales?

Apparently the old method of creating i18n folder inside instance home
does not work for overriding translations in locales, so what does?

Someone mentioned that adding some zcml to register another directory
for i18n stuff could work, but the post did not give any specifics nor
examples. Is that how it is done - what's the syntax and which zcml
file to modify - and what's the folder/file structure for the override
directory holding the new translations?

Thanks!

 Petri

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