It makes sense that much of the Midwest is dominated by German, as over 50 million Americans claim some German in their bloodlines. The large population of Filipino immigrants on the West Coast are responsible for the spread of Tagalog. Michigan, home to the nation's largest concentration of Arabs, easily claims Arabic as its third most popular language.
But, notably, the number of people who speak German, or Tagalog, or Arabic, is much, much smaller than those who speak English or Spanish. Saying that "German is the third most-popular language in the Midwest" might be accurate, but it's still doesn't come close to the number of English and Spanish speakers. In 2000, there were just over 1.3 million native German speakers in the U.S.
Blatt also breaks down the prevalence of Native American languages by state, with Navajo as the leader, as well as Scandinavian, Indo-Aryan, and African languages. Check out the rest of his maps here. (Slate)