Add translated labels in Plone core

i was fixing some translations for Plone5 and i've found that some labels are untranslatable (simply because they don't have any msgid).

I've found some old instructions that tells to run buildout with "-c experimental/i18n.cfg", but that cfg seems to be a bit old (for example it still use LinguaPlone).
I tried to use the generated script, but it doesn't work: i added a new msgid for plone's domain in Products.CMFPlone, but the *.po file in p.a.locales isn't get updated.

My question is: what's the "official" way to add some core translations? This script is still valid and i'm missing something or there are other ways to do it?