Justin Bieber's seemingly masterful Spanish singing on the #1 hit "Despacito" did not translate in a live performance ... unless we missed the "blah blah blah" lesson in Español class. 

The Biebs got on the mic Tuesday night at 1 OAK in NYC, where he tried singing along to Luis Fonsi and Daddy Yankee's hot record -- which features Justin singing the hook in Spanish.

It appears Justin forgot the more intricate elements of the chorus, and by intricate we mean everything but "despacito."

The takeaway here -- Spanish is hard when it's not written down for you in a studio.