Another thing that might be noteworthy:
In gaming terms a "level" is not usually the same thing as a "chapter", "stage" or "map".
In some games like Super Mario Bros. in which your character doesn't really change his skill set you can often interchange "level" with "stage". In other games this is a big difference though!
In more complex games your avatar/character might have a "level" (for example: "My Barbarian is now level 49 and I only need a few more XP to get him to level up to 50.") and you either play through a storyline with "chapters" or you play a "stage", an "area" or on a "map".
Make sure that "level" is the correct word to begin with 0815clodette.
"Would you like to buy additional stages?" means more game content.
"Would you like to buy additional levels?" can be taken to mean more game content (eg. more stages/areas/maps) or paying to level up (which is not unheard of).
Only use "level" if the context is very clear. I'm only mentioning this as you've said that the original text is Korean and you have already corrected a few mistakes in the original.