German stealth invasion.

I know, that surprised me, not even sure I trust that data.

The data is self reported to the U.S. Census:

http://www.slate.com/articles/arts/culturebox/2014/05/language_map_what_s_the_most_popular_language_in_your_state.html

Note that the mapmaker himself has a healthy skepticism, too.

That's part of why he did this exercise:

Last month, I wrote about the fun and the pitfalls of viral maps, a feature that included 88 super-simple maps of my own creation. As a follow-up, I’m writing up short items on some of those maps, walking through how I created them and how they succumb to (and hopefully overcome) the shortfalls of viral cartography.

One of the most interesting data sets for aspiring mapmakers is the Census Bureau’s American Community Survey. Among other things, that survey includes a detailed look at the languages spoken in American homes. All the maps below are based on the responses to this survey. However, an ACS participant does not select his language from a list of predeteremined options; he fills in a blank box with his self-selected answer. For instance, some people answered the ACS with “Chinese,” while others gave specific dialects such as “Mandarin” or “Cantonese”. These were all treated as different languages in the ACS data and when constructing these maps. (See the raw data here.) New York is marked “Chinese” because more people responded with “Chinese” than any other language other than English or Spanish. If all Chinese languages (or languages under the umbrella of a larger language family) had been grouped together, the answers for many states would change. In addition, Hawaiian is listed as a Pacific Island language, so following the ACS classifications, it was not included in the Native American languages map. The spelling of each language is based on the language of the ACS.*