Wow that weapon in-game refers directly to the spear of the bible, I didn't know.
Haha yeah 4 is nice ^_^
Longinus would be its name IF it took its owner's or maker's name, à la Masamune, Murasame, and so on.. but that doesn't seem to be the case here, Wikipedia says Spear of Longinus, and in Latin Lancea Longini (literally Lance of Longinus). Yeah, "Longinus" doesn't fit.
Let's discuss..
Holy Lance seems to be the most common name for this biblic weapon.. that could be a good argument
but don't the kanas point towards Longinus ?.. then yeah, I would not be surprised at all that the intended then shortened name was Spear of Longinus
Lancea Longini.... for me, that is not like a proper noun, the item's proper noun, but just "Lance of Longinus" in the original language...
Because of the kanas, I would go with Spear of Longinus.