If only the government “decided”. no one will care and Americans will continue using English as usual.

If all Americans decided to speak Spanish. World would still continue to speak in English because currently all information in all fields are mostly in English.

If all Americans decided to speak Spanish and translated everything into Spanish then probably world will start speaking Spanish.

A real life example is when Romans conquered all Greek territories, all those nations continued for decades writing in Greek.

I know for a fact that in Egypt all taxes and government related stuff remained in Greek even decades after Arab conquest.