My grandmother tells me the story of how mad she was when she started going to kindergarten/1st grade and she couldn't speak English yet.

The footnotes on page 11 give a good explanation.

For the most part, Austria, Hungary, and Poland are broken out separately according to their present borders, even for the periods when they weren't separate countries.

He rode the bus to work and it wasn't uncommon for women mostly to come up to him and physically attack him wondering why he wasn't in the armed forces.

I asked my grandmother once why she didn't teach either of her children to speak German. She told me that it was on purpose, they were ashamed and didn't want to make them targets.

I can readily believe that more immigrants arrived from Germany than from the British Isles, but it's also possible that by the time those German immigrants showed up they were outnumbered by the descendants of the earlier immigrants from the British Isles, perhaps even just immigrants from England itself.

I've never seen any sort of analysis like this, though.

German used to be everywhere, being heard in the streets, with large numbers of German-language newspapers and cultural clubs. The Germans, being Germans, liked to organize themselves into clubs.

It gradually died out, as immigrants stopped teaching it to their children. Usually the children of immigrants learned the language at home, but they didn't usually pass it on to their children. World Wars I and II did a huge amount to suppress German culture and language, but it would have eventually died out anyways.

By World War II, German was still common enough that the Army was able to find large numbers of German interpreters among the young men who had spoken it with their parents.

My grandmother knew German as a child, but the language in the family had switched to English by the time she was about 10 years old. Once she was grown, she wasn't married to a German speaker, and hadn't spoken it in years anyway, so none of her descendants learned it. By the time I talked to her about it in her 80s, the only German she could remember were a few lines of some children's poem she recited when she was a child.

I suspect that this will eventually happen to Spanish, although the continued migration of Spanish-speaking immigrants will slow down the process. Eventually, the non-Spanish-speaking descendants will greatly outnumber those who speak Spanish, and the language will slowly decline.