xml is good, and human readable. it also has a concept of schemas.
Now have game content driven in xml and schemas used to validate, so pretty much have enough information from the schemas to make a generic visual editor for xml documents.
and voila, i have the first generation of game data editors for my game....
first c# wpf app, pretty nice tech to work with, at least quite to develop .c# feels halfway between c++ and python, each of which i like, not so sure about c# though)
alows editing of properties (string, bool, float, file path, colour, enums) and inserting/removing/cloning xml nodes, with the schema dictating what are legal nodes to insert/ remove.