He obviously knows some vocabulary, and honorifics are important enough in the culture that he might learn their proper use BEFORE full conversation. Intruducitons are kind of important after all. As well it should be safe to assume that he is into Anime enough that he could easily have tried using Chan on Nanase and got shut down while they were dating.
The lack of confirmation does not mean a negative. It is possible he learned, his Aunt may have taught him for all we know, but it is also likely that Edward deliberately did not teach him. Of course once you factor in regular contact with Will and Gill (as well as possibly other Aliens) his range of potential languages spoken is extreme. It comes down to how much interest Tedd has in his heritage.
Which means she can not negate her, until the story confirms or denies something it is still a possibility.
The amount of overlap in what we do know that the sell with anime fans and otaku is large enough that it becomes a real possibility. Pretty much every card tournament I have ever gone to there was at least 1 group of Anime fans there, and they would discuss things about the shows. There is enough cross over that it is something Justin might have learned somewhere. I had listed Elliot and Justin as they are the 2 Susan is most likely to interact with, other then Sarah who seems more into French and european languages/culture then japanese to me.
Subtitlers tend to leave in the honorifics, as huge a anime nerd as Greg is then he is likely to know something about them.
Why would there need to be? It is strongly implied that he is into Anime however.
Really the entire "there is not hard evidence so you are wrong" line of thought is the worst type of argument for this sort of thing. Characters are never fully fleshed out unless the story ends up tracking every aspect of their lives which they never do. All we know is what we have been told, with everything else a possibility, the more something relates to what we do know about a character the stronger the possibility. All i was saying is that there are cases to be made for most of Susan's circle of friends and acquaintances.
Greg is a stretch, but we have a 6 month gap in the timeline with almost no information.