Thats the korean language, you can change it through Tools > Settings > Display Encoding to Default
And the grf you open that error, a sprite must be .act and .spr
.act is for the action, size, color, rotation, etc
.spr for the sprites image
the error you got means there are no .spr because chibi grf only change the size of character (it still use original Korean RO character sprite, weapons, etc)
but if you want to change back some or all character to real size, just unload the grf from data.ini or maybe you can delete the .act in chibi.grf