What manga hasn't been localized yet?

[Kuukai]

GnU was never localized because it never had a tankoubon. We're also missing Quantum+ and Quantum i because they're new/ongoing. Other than that, every manga series made it over. There are various 4koma and other strips scattered throughout various publications if you mean everything that's technically manga, though.

[Erroneous]

Are you just looking for the Mangas or both the mangas and the novels? Then obviously ZERO and Epitaph of Twilight would be in the novel category. Then the remainder of the last ones localized. And I never bought the Rena special pack so I have no idea if its a novel, manga, or neither.