Giving a polite response, with the use of "sonna ni"

[bunalz3]

Person A: Eigo wa tokui desu ka?
I really am not quite sure whether he meant (1) Are you good at English?, or (2) Is English your native?
So far, I'm guessing it's the former.

You can say that Person B is actually good at English, but Person B wanted to answer that modestly... so the following are what I could come up with:

Alternative 1
Person B: Sonna ni umakunaindesu kedo, doushite? (Note: "doushite" is included since Person A asked the question all of a sudden)

Could the above mean "I'm not really that good..." (I wonder if "that" and "sonna ni" in this context mean the same thing)

Alternative 2
Person B: Betsu ni, doushite? (This one sounds kind of rude since it doesn't actually mean "Well, not really...")

Alternative 3
Person B: Well, I guess so.../You could say that... (I absolutely have no clue as to translate these ones--まあ、そかも/かもしれん--which sounds obscurely funny to me )

[Ongakuka]

It means the first, yes you can interpret it as 'is English your better language' if it is clear that you speak multiple languages.

To reply modestly, you could perhaps include the phrase 'kanpeki dewa arimasen' (it's not perfect)

EDIT: If you really want to use 'sonnna ni,' you could say 'sonna ni jouzu dewanai to omoimasu.' I think 'umakunai' is quite casual but it's your call

[bunalz3]

Ah, I see. Thanks Ongakuka-san!