The keyboard that's included in GrapheneOS is actually the Android Open Source Project keyboard. From what I read, it's mostly abandoned and only updated by Google when it breaks. So, you would need to install a different keyboard to type in another language.
I'm not familiar with Tibetan, so I don't know which keyboard is best for you. Google's GBoard and Microsoft's SwiftKey keyboards both seem to support many languages. I don't know of any open source keyboards with broad language support, so you might have to search around to see if any have support for Tibetan.