National Language is known to maximum people of that nation. Official Language is the language used in government purpose...

This question arises only in the case of multi lingual nations which together have formed a sovereign country. The former USSR for instance. A good number of the republics that formed the USSR had their own national languages. However to the best of my knowledge Russian was the official language of all the Republics as a matter of compulsion from Moscow. After the disintegration of USSR the independent republics have adopted their own national languages as their official languages too. You can take the example of India which too has a large number of languages spoken by people in different parts of the country. Infact most States which comprise the Republic of India are constituted on a linguistic basis. Almost all the States have their local language as their official language. The Indian Constitution also recognizes all major regional languages as national and official languages. However at the national level for administrative convenience Hindi and English have been adopted as the official languages for communication purposes. Hindi because it is the language understood by the largest chunk of the population and English because it is the only language that people in all parts of the country understand. So in many cases the difference between the national language and the official language are somewhat blurred. The Indian Model has been working somewhat satisfactorily for over 60 years now.