Any analysis of American children that ignores demographics is silly. 44% of California children don't speak english at home. What on earth do you expect from these children if you test them in english proficiency?
irrelevant: there are no hard numbers for that and that's not the timeframe the article is discussing (since the 90s)
Also I'm quite sure the numbers are there, maybe not as crisp as you like, but one could make some reasonable estimates based on immigration records from ellis island and the census through those periods.
Which would also be 1918 which is first world was and results back then were likely to he affected by that big time. And it was also time of racial segregation which would affect a lot too.
The kids section at the local library branch (there is one in every neighborhood) will have half the books in English and the rest in whatever languages are common in that neighborhood.
Language barriers are only a problem if your local city makes them one.
My son is in 3rd grade and his NWEA MAP reading score put him at the 11/12 grade level, so he got put into an advanced reader class. In the fall, they read books about history, wrote about and discussed them in class. This spring, the teach chose a young adult novel that I guess is entirely appropriate from a reading-level point of view. But some of the subject matter goes way over the heads of the kids and other stuff in the book is disturbing to them. Our son simply refused to finish reading the book. The teacher wasn't going to switch books because a couple kids didn't like it, and the class depended on discussing what they read... What can you do but stop reading and/or drop out of the class?