A National language was the language given special status by a nation, usually the language spoken by most members of that nation.
English as a national language
The Bartlet Administration was against legislation that would make English the national language. In 2000, Steve Onorato, furious about the President's recent FEC appointments, stated that a bill proposing the national language would be the first among many bills passed by the Senate in retaliation for the embarrassment of the appointments. (Let Bartlet Be Bartlet)