In "The Expression Amrilato", the main character is lost in an alternate world where Juliamo (which in the real world is Esperanto, an existing constructed language.) is the official spoken language. Therefore, while enjoying a story between cute girls, you can also learn the Esperanto language! The text in this game has been officially supervised by the National Esperanto Association in Japan.
You additionally might be able to learn Japanese in the Japanese version included in the package, as well as Chinese in the Chinese version planned in the near future!?
The game's release on Steam has been delayed until further notice but it is available on GOG and through MangaGamer's store.