I feel like I would have benefited from learning at least some the actual numbers before having decade and century thrown at me, since decade is blatantly year-tenty* and century is clearly year-hundredty*. ;_;
(* No, neither tenty nor hundredty are real English words, but hopefully you get the concept!)
"decade" is a false friend in French. "décade" means ten days. (from Latin decas/decadis, from Greek). It was used during the french revolution to replace the week. The french word for "decade" = 10 years, is "décennie". Although, more and more people are using "décade" for a period of 10 years, due to the influence of English. The same phenomenon is occurring in Spanish (decenio/década), Italian (decennio/decade) and Portuguese (decênio/década).