If Linear momentum is defined as 'mass x speed', can we state a principle of conservation of linear momentum as 'linear momentum of a system remains constant if no external force acts on it?

Yes. But Not almost . A little Bit Of Addition Of Sum terms of great Importance Should be Added To your Statement.(vector sum of linear momentum)

The principle of conservation of linear momentum states that,

If no external forces act on the system of two colliding objects, then the vector sum of the linear momentum of each body remains constant and is not affected by their mutual interaction