Why are they called Adeptus Astartes?
Hello, Astartes means Stars, Adeptus means the ones that get hold of something. So the Adeptus Astartes means literally those who conquer the stars aka Space Marines.
Is 40k Latin?
It's not Latin, it's Gothic. High gothic for upper class and low gothic (and its variants) for the teeming billions. The old explanation was that it isn't Latin but that's just a representation of a complex formal language to let readers know it's not the “common” speech.