Rokkuhato is the japanese pronounciation of Lockhart. 'Lock' tranlates roughly into Rokku, because L is pronuced as R, and they always have a vowel at the end of a word except when it ends in the letter N. And 'Heart' translates roughly in Hato for the same reason. They aren't tranlating the meaning, they're tranlsating the sound of the word as best they can.
Also, Sora Light and Kairi Love? I lol'ed at that. If they had those surnames, I'd die laughing, and then change their names.
And you said that the english version SHOULD have english last names. Oh yeah? *Goes through names in Tekken 5 manual* all of the japanese names aren't translated. Asuka is still Asuka, Heihachi is still Heihachi. And the ones with english names had english names in the original, like Anna and Steve.