Slime Forest is a work in progress, new versions usually come weekly. Currently, what you'll learn with this game is kana and some kanji.
Outside battle, Slime Forest is just like any console RPG. But in battle, your enemies (slimes they all) will throw random characters at you. If you type in the correct transscription/translation, they are defeated. If not, the answer is displayed, and they grow stronger.
The first time a character appears, you simply have to guess. For all kanji, there are mnemonics which are sometimes quite absurd, but usually helpful. (If your samurai gets wet, you have to hang him up to dry - how else explain that the kanji for samurai, when turned upside down, becomes the kanji for dry?)
As a game, Slime Forest is often rather odd. You level up, but your hitpoints don't increase. You save and quit the game by resting at the inn, and the outcome depends on how soon you start playing again: if you do it too fast, you won't heal, if you wait too long, you will be charged for overstaying. But Slime Forest should not be seen as a game. As a tool for learning Japanese, it's one of the best there is.