When do you think Luke learned to understand R2D2?
Luke clearly doesn't understand R2D2 in the OT. His X-Wing has a translator, and Luke clearly guesses what R2 is conveying while on Dagobah.
Anakin also had his ship translate for R2 in the PT.
In Disney Star Wars, it seems everyone can just understand droid-speak, including Luke. You think the Republic made learning droid-speak mandatory?