Codeacademy is nice, but if I were you, I'd study the ultimate Japanese site in the entire web: alljapaneseallthetime.com . Gamification is nice, but your customers already are motivated to learn Japanese. They just want something that can effectively get them up to speed, and increase their confidence. Rather than have points/badges, tell them they just learned 50% of the most popular Japanese words after 1 week.