These two flags would have been used for the Japanese and Chinese language selections on the main menu. However, the main menu doesn't allow one to select either and these. These likely would have just changed the help text.

Gudeballs HiddenPic1

Gudeballs HiddenPic2

Gudeballs HiddenPic3

These three frames are hidden in the game. While the first picture can be seen by entering 9999 as a password, it's unknown how to see the other two.