Regis Baccaro shows how to change the user language in SSRS 2016:
Changing it to Spanish and refreshing the browser changed the SSRS user language to Spanish without me having to add a language at all.
To change the user language of SSRS 2016 you need only to change the Formatting Region setting from the control panel – nothing else !
The issue I tend to have with this is that different tools tend to behave differently when you start changing the format settings. I recall Excel being particularly finicky about it.