Is there a offline version of askubuntu.com? Can I download it as in Wikipedia? Or is there any app that mirror it ?
All Stack Exchange sites, except beta sites, have a data dump provided by torrent, about every six months originally and recently on a more frequent basis (sometimes every month, sometimes every few months, as far as I am able to see). When they're not listed there, you can find them by searching on ClearBits.
The most recent data dump was taken on March 2013 (just last month), and is available here.
There are a variety of ways to search, browse, and view the data; one of them is this reader by Nathan Osman. If you're interested in processing the file yourself (or exercising a greater degree of control over how it is processed), you should take a look at this MSO question and that one too.