We made a copy of the live site for making a staging site. We did the updates and everything is working fine. Except for te translations not working (anymore).
Under 'CiviCRM » Localization » Languages, Currency, Locations' we set Default Language to Dutch (Nederlands). After the updates we saw that in the Civi module folders "l10n" and "sql" where empty. So i copied the files back from the production environment en put them back.
From documentation, this should be it to make it work but everything is still in english. After searching for a full day i don't know where to look anymore. Resource url should be ok and no other errors on the site.
Searching online for this problem does not give much results.