Try to follow the conversation between somebody living in my village and the next one 5 km further down the road. Or a bavarian and someone from Hamburg. They wouldn't understand each others dialects. You know what I mean. Even the folks down on the mexican border get fox news and understand plain old english.
Things tend to be pretty ok in western europe where most can agree on english, but don't forget that a large part of europe was behind the iron curtain and didn't get to learn english until 1989. The majority of people in the east doesn't speak any english at all.
In the city I named (Watsonville) there are many people that don't understand English and many of the signs there are in Spanish. This city is in the heart of agricultural California so most foreigners (and Californians, to be honest) never see it.
It is only about 60 minutes from Silicon Valley and yet has more culturally and linguistically in common with Mexico in many ways.
The people 150km to the south from here speak Sorbisch [1]. They have their own culture, their own schools even though they're mostly german national. Still, they get german TV and mix with "regular" germans on a daily basis. However, the people 200km to the east all speak polish and don't get any german tv. Which group is more likely to have some common ground with me?
The question is not if there are enclaves of other cultures and languages within a larger body of uniform people since on a small enough scale you will always find enclaves: Chinatown, Little Italy, or Kreuzberg in Berlin[2] but how uniform the population is. No population is truly uniform and I don't think anyone pretends that West-Coast Americans and East-Cost Americans are the same as Texan, but I dare to postulate that a common overarching government and a common official language leads to a greater degree of uniformness than 20+ official languages and a loosely knit set of contracts. A norwegian is most likely more different from a greek than a New Yorker from a Californian.
[1] The Sorbs are one of a couple recognized german minorities which explains their special status http://en.wikipedia.org/wiki/Sorbs
[2] Strong turkish/arab minority
Things tend to be pretty ok in western europe where most can agree on english, but don't forget that a large part of europe was behind the iron curtain and didn't get to learn english until 1989. The majority of people in the east doesn't speak any english at all.