A person is identified as belonging to a particular nation by his cultural traditions and the language that he speaks. The language as well as the traditional practices usually develop and get established over a long period of many hundreds of years.
They give an identity to ah individual wherever he is. For instance, a Frenchman will normally speak the French language fluently. He will also follow French traditions and customs wherever he is in the world, as he would have imbibed them in his family from his childhood days. Thus, he will be identified as a French national.