Actually, I can't find any scenario except for the validation error , in my opinion, the validation error will show for the end user(which use our app), they may not understand  any english. so "please enter integer for age" will translate to "年龄必须为整数"(年龄 means age). if the age parameter is not translate , it will become "age必须为整数", then the chinese user will confusing, which form filed is wrong?
Or I'm wrong about the grape validate, it's for the user who using the api, more like a documentation?