civilization : why is language considered to be the treasure of a nation civilization

Native language is the main treasure of every nation.

Because you have feelings for your nation's own native language. As you have a feeling about your nation that, this is your nation you feel to be proud to say yes this is MY nation.

Same feeling works with the native language you feel proud to say yes this is MY language and it connects you with the rest of the people, their wisdom, their history.

So the Native and the Native language is like the main treasure of every nation.