Is it true that some pokemon like darmanitan and oranguru can learn sign language?

Absolutely. Actually, any ape-like pokémon—even those only marginally apes, such as slaking—are capable of learning sign language. All of them have strikingly human intelligences … as well as the appropriately shaped hands to use with most forms of human signing. It’s quite interesting, actually, and the subject of ape pokémon communication has been the focal point of pokémon behavior and intelligence studies for the past five decades. There was even a rather famous darmanitan named Koko who was known for having lengthy, meaningful conversations with her trainers completely in ASL. I would highly recommend doing a bit of research into Koko to learn more about ape pokémon communication, should you be interested.