While I love Earthsea and its magic, I don't think it's a good match with D&D 5e (which I also like, mind you.)
If I had to come up with a quick and dirty implementation, I'd simply say that
- the true name of things and entities that don't have a stat block (a pebble, a single blade of grass, a cup, a cloud, etc) is "included" in knowing the (official D&D 5e) spells that affect them. If you mend a broken shelf with Mending, you used the true name of the shelf, and that's it
- not knowing the true name of statted things and entities but trying to affect them with spells works as per the official D&D rules
- knowing the true name of statted things gives you an advantage on any roll you're required to cast OR a disadvantage to their save, OR maximum damage / healing / duration rolls etc, noting that learning the true name of a statted thing is the equivalent of gaining and possessing a magic item.
And that's it. Anything deeper than this, I'd go look for a different system more suited for Earthsea's world and magic.