Before launching your game please make sure to set up your preferred controls by going to the "Game configuration" menu. You will also be able to set your preferred language here:
Please note that the Load/Save/Quit Game menu will be by default assigned to the "Enter" key.
If you are using GOG GALAXY simply select the Customize icon on the right of the Play button ->Additional executables -> Game Configuration
If you aren't using GOG GALAXY simply go to the game's installation folder. Default:
C:\GOG Games\Metal Gear
Once here launch the "settings" executable: