Is it just me?

As far as I understand, yes, they are speaking German. Mennonite (and Amish and Hutterite) colonies have been in North and South America for many decades, so their dialect has evolved and they wouldn't sound the same as anyone in Germany. They are speaking Low German, or Plautdeitsch, but have had no contact with other German speakers for generations.

This is from Wikipedia:
"Plautdietsch is spoken by about 400,000 Russian Mennonites, most notably in the Latin American countries of Mexico, Bolivia, Paraguay, Belize, Brazil, Argentina and Uruguay, as well as in the United States and Canada (particularly Manitoba, Saskatchewan and Ontario)."

Although there are many Mennonites, at least in Western Canada, who are completely assimilated into mainstream society, there are also separate colonies like the one shown on the TV show. In Saskatchewan, I've heard assimilated Mennonites call the ones in the colonies "Old Colony Mennonites". The Old Colony people whom I've seen in Saskatchewan don't wear all-black clothes and black hats like on the TV show; the men all wear blue jeans and plaid shirts and there aren't very many hats, and the women wear light-coloured flower-print dresses, but they do pin their hair up like on the TV show.

The TV show portrays the Mennonite colony as being very insular, which may be the way they do it in Ontario. I assume that they call anyone in mainstream society an "auslander" or foreigner because they view them as a challenge to their way of life.

I agree that the plot about hardcore drug dealers being Old Colony Mennonites is a bit weird, but I'm willing to keep watching the show and see where it goes.

Being German, you likely speak High German. That's what they speak in Germany today.

Mennonite German is a dialect of Low German called Plautdietsch.

Just like English went through the great vowel shift rendering Anglo-Saxon (Old English) pretty indecipherable to speakers of modern English, so did High German.

High German underwent a vowel shift and Low German did not. So they sound similar, but not the same.

Many languages share a history and sound similar in some ways. Like how I can understand words like "Bier" and "Alarm" and even "Apfel" despite not speaking Standard/High German.

The questions on the accuracy of the varieties of Mennonite cultures across Canada and the world is something I can't answer. The Mennonites I know are in the city and have jobs in things like IT and business.

I watched the first two episodes. Since it's only 6 episodes I'll finish it. This gives me my Canadian fix that it seems like I get once a year.

The acting is a little off, but it hooks you in in a way.

It's very similar to The Path, but there are worse shows you can copy.

I just wanted to point out that the show is based on real events. There was a Fifth Estate documentary done on the Mennonite Mob in '93, and there continues to be a problem with drug lords within Mennonite colonies