I see your puny 99s and give you 100 percent: Powder River County, Montana. It even has a Boot Hill Cemetery, established 1876 at Deadwood Crossing. The HQ of Cross Ranch in the county was added to the National Register of Historic Places in 1996. The lovely town of Broadus lies in the center of the county. According to the town web site:

"We are located in southeastern Montana on the Powder River, near Custer National Forest. In the Powder River Valley, below the Little Big Horn Mountains, along Highway 212 - the route to the Lewis and Clark Trail and Pompeii's Pillar, between the Black Hills, the Battle of the Little Big Horn (Custer's Last Stand), Yellowstone Park and heading toward Glacier National Park, sits Broadus, Montana... Your home in the hills, it's the perfect base camp for your traveling adventures. You'll find yourself at home in the famous Judges Chambers Restaurant and Broadus Motels family owned by Jean Hough and son Stephen Held. Traveling out each day, you'll return to the finest food and accommodations, at a fraction of the cost in the surrounding area.

"Broadus lies in the center of Powder River County and is home to some of the finest wildlife in North America. Year around wild game abounds, making it a prime spot for photographers, family outings and hunting. With hundreds of thousands of acres of National Forest and BLM in the region, along with private ranches and hundreds of miles of accessible roads there's no end to your scenic pleasure.

"Located in the midst of several Native American reservations you can get to know these people as they are today. The Powder River Valley was considered the last great hunting ground of the natives. It's common to find arrowheads and artifacts as you hike through these ancient lands.

"Dinosaur enthusiasts won't be left behind. Many of the world's finest skeletal remains were discovered in this former coastal region. The mighty T-REX was the ruler of this land millions of years ago. It's possible to find eerie reminders from many of the giants of that era."

I stayed at the town motel once. It had cable TV and shared bathrooms between rooms...

Forgot to add: "Our deep sense of community and Western hospitality has given us the reputation of being the 'wavingest town in the West!'"

I bow in deference to your impressive find. Indeed, the census report shows 100%: Powder River County's 1725 residents are all English speakers.

I'm a little concerned about visiting, though. From the same website you quoted above, "If you are interested in experiences most modern people don't have, consider calling varmints like coyote, fox, bobcat or the illusive mountain lion. With local ranchers reporting increasing loss due to these predators, your chances at a shot have never been better.... perhaps you'll find your way into a plane with professional predator control experts."

But they always wave at the lions before they blow their heads off...