It is a German dialect. According to a Mennonite fried who spent some time in Germany it is close to a Southern German dialect that is considered hick.
German, English and Dutch are in the same language group: West Germanic languages, group of Germanic languages that developed in the region of the North Sea, Rhine-Weser, and Elbe. Out of the many local West Germanic dialects the following six modern standard languages have arisen: English, Frisian, Dutch (Netherlandic-Flemish), Afrikaans, German, and Yiddish.
If one of those languages is your native tongue, you can learn the others relatively easy. Although they are totally different languages.
And all of them have various different dialects. So it is hard to say what the Amish exactly speak.
Over the time languages evolve. Afrikaans started as 16th- 17th centuries Dutch. But both languages are quite different now.
Same goes for the Amish Dutch\Deutsch (German).
My regional dialect is clearly a bridge between German and Dutch.
Dutch is my native tongue. I'm more or less fluent in German, English and the Limburgian Dialect.
I can understand written Afrikaans.
I understand almost all German and Dutch spoken dialects, can't speak them though.
Flemish is no problem to understand as long as they don't use Flemish dialects.
I must be and descendant from one of the Germanic tribes.
Most likely from the Saxons.