FYI. I sent an e-mail to the Migrationsamt in Zürich, obviously in my best German 
, asking whether the renewal would be subject to the new language requirements. Quote:
Die Voraussetzung bzgl. Sprachzertifikat gilt deshalb nur für die erstmalige Erteilung der Niederlassungsbewilligung, nicht aber für die Verlängerung der Kontrollfrist.
Obviously this is “just” a response from a civil servant working there, i.e. it has no legal value. However, for me it’s enough not to do a language course before I receive the Verfallsanzeige, which I presume will have the exact requirements.