there isn't one
Asia is a continent not a country so it doesn't have an official language. Or rather it has many official languages...
Cambodia
In East Asia, English is only official in Singapore and the Philippines.
I don't think Asia really has an official language because it is a continent, and the largest continent as well. There are many different sovereign nations on that continent. No one is going to tell people from one nation that their 'official' language is different from their native language.
Typically, the official language in the Caliphates was Arabic, but there were several Caliphates in Persia, Central Asia, and India which did not use Arabic as their official language. The Ottoman Empire also used Turkish.
The Philippines is the only country in Asia where Spanish is still spoken, although it is not an official language. Spanish was introduced during the colonial period and has influenced Filipino culture and language.
Malaysia,Philippine,Thailand,Singapore
There are more than three countries in which Portuguese is the official language. For it's the official language of the southwest European country of Portugal; and of former colonies of Portugal in Africa, Asia, and South America. In Africa, Portuguese remains the official language in Angola, Cape Verde Islands, Guinea-Bissau, Mozambique, and Sao Tome and Principe. Additionally, it now shares official language status, with Spanish and French, in the African country of Equatorial Guinea. In Asia, Portuguese shares official language status in East Timor, with Tetum; and in Macau, with Chinese. In South America, Portuguese is the official language of Brazil, which is the continent's largest country. And so there are ten countries in which Portuguese is the official language.
Russian is officially used in four countries:RussiaBelarusKyrgyzstanKazakhstan4 countries have Russian as an official language: RussiaBelarusKazakhstanKyrgyzstanRussian is also official in these regions:Crimea, UkraineGagauzia, MoldovaTransnistria, Moldova
Russian was the most common foreign language in the countries of Central Asia in the 1970s due to the historical influence of the Soviet Union, which used Russian as the official language.
Portuguese is the official language of former colonies of Portugal in Africa, Asia, and South America. In Africa, it therefore enjoys official language status in Angola, Cape Verde Islands, Guinea-Bissau, Mozambique, and Sao Tome and Principe. Additionally, it now shares official language status with Spanish and French in Equatorial Guinea. In Asia, Portuguese shares official language status with Chinese in Macau; and with Tetum in East Timor. And in South America, it still is the official language of the continent's largest country, Brazil. Additionally, though not official, Portuguese is one of the nine recognized regional languages of importance to the neighboring South American country of The Co-operative Republic of Guyana, where English remains the official language.
In Latin America, Brazil is a former colony of Portugal, and has Portuguese as its official language. In Africa, Cape Verde Islands and Guinea-Bissau likewise are former colonies, and keep Portuguese as their official languages. In Southeast Asia, East Timor is a former colony, and maintains the official status of Portuguese right alongside of the indigenous Tetum language. And in Asia, Macau is a former colony, and uses Portuguese and Chinese as official languages.