Great news fellow TS fans!
The sixth season of Totally Spies! has recently been announced by French television network TF1 in an online article! The original quote: "Quant à la série phare de la chaîne, Totally Spies, elle reviendra en 2013 avec de nouveaux épisodes survitaminés." (English: "As for the [TF1] chain's flagship series, Totally Spies, it will return in 2013 with new, super-charged episodes.")
Additionally, as part of a new television deal, Cartoon Network Latin America has licensed all 6 seasons of Totally Spies (156 x 26 minutes) and Totally Spies: The Movie (1 x 75 minutes).  Though it is currently unknown what new plots and villains the Spies will encounter in the show's sixth season, there will be 26 episodes in Season 6 following their ongoing journey in the exciting and dangerous world of international espionage.
The exact date on which Season 6 will begin airing has not yet been publicized, but it will happen sometime in 2013. Check your television schedules regularly and keep an eye out for any future announcements! Judging by the overwhelming number of Totally Spies! fans currently on Facebook, it isn't an exaggeration to say that Season 6 will be the most anticipated season of TS in the show's history. 
You can discuss Season 6 here