Yeah, the big ones are obviously Hutterite colonies in the Dakotas then. Though it's also true that the "Germans from Russia" came to the US later than Germans from Germany proper and were in more isolated settings. Their late arrival in the US and isolation would have made them less susceptible to the anti-German hysteria of WWI.
Still I find it remarkable how long it held out. I think it's probably reasonable to assume that most of the second generation was born in the first quarter of the 20th century and the third generation in the second quarter. So people born before 1950, say, may have grown up speaking German (and the older people are still speaking it in their homes today to some extent).
The Germans from Russia, or the Volga Germans, settled in both cities and rural areas. Concordia University has detailed descriptions of the
areas and years of immigration which amounted to almost 120K by 1920. As a descendant of the Chicago immigrants I can say that though the first generation spoke German at home their children, including my grandmother, did not and I only heard snippets as a child. According to
this source the assimilation was driven by concern over the Russian connection, not the German one.
