According to a book I am reading, the Chinese always had a line of thinking that involved "balance": The majority of your meal is to be a starch and the starch is then flavored with a small amount of vegetables and meat.
I think most ethnic diets are based on what resources were/are available to that culture, since the current abundance and variety that most of the First World has is a recent phenomenon. I think you will find that many ethnicities actually have more of a variety of nutrients in their food than it may appear at first.
In particular, it is common to associate protein with meat, calcium with milk, and omega-3's with fish: Most foods are a complex mixture of proteins, fats, and carbohydrates. Whole grains, for example, provide proteins as well as carbohydrates. Leaves such as kale, spinach, and chard are valuable sources of calcium, and omega-3's can be found in walnuts, purslane, and flaxseed.
I don't know where you're located, but I think many Americans eating at restaurants get a rather incomplete view of what people from other places eat, because we end up with only a subset of the dishes that a culture eats and most dishes are modified for the local palette and availability of affordable ingredients.