Unfortunately not. Lot of people asked for this and discussed with the "developers". They say its hard to implement, while preserving their other goals of providing Leaderboards and Scores (you get points for each achievement). With offline achievements its easier to cheat them. You need an account too, otherwise it would not sync to the online database. I wish there was a way to download and use it without an account, so its not possible to cheat, because I personally don't care about the online Leaderboards. This is the official reason.