But there are only two languages with large demographics in the USA: English and Spanish. If the FCC created a regulation that said streaming services must support at least 30% Spanish, I think that would be an improvement, and it would also be easy for the streaming services to hit that goal.

Perhaps you could argue that smaller language groups should also be supported. According to the 2010 USA Census, there were 2.9 million people in the USA who spoke Chinese. I think proportional representation could be useful, though clearly you could not give each language group 30% of the streaming service. But if the regulation insisted that the movies streamed had to match the languages, proportional to the demographics of the USA, I think the streaming services would be the better for it.