Adding, editing, planning tasks can be easily achieved in Ubuntu with Nitro, a "young" yet fancy application that brings a clean, properly organized interface with support for list, sublists, "live" search, logbook and more.
Nitro 1.0.6 has been released, adding support for Simplified Chinese (along with English and Pirate English), thus extending its language support, language that can be selected via the new language selector (Nitro-->Settings-->Language).