...has anyone noticed that the show has placed a great emphasis on certain cultures (and less on others)? This seems reasonable, given that many of such cultures are quite prevalent in pop culture and everyday life, but here's what I've taken note of.

I've noticed an abundance of emphasis on/references to the Japanese and Turkish cultures (even outside of the context of Adil's life), especially in the middle & later seasons:

1. The Perfect Game: The Hagia Sophia is one of the buildings Mr. Ratburn mentions.
2. Kung Fool: In the intro, Mr. Read prepares a Turkish dish of stuffed grape leaves for dinner.
3. Get Smart: Mr. Ratburn asks HUGO a question about Ottoman puppet shows.
4. The Frensky Family Fiasco: The Frenskies eat sushi.
5. Flea to Be You and Me: It's revealed that Mr. Read has a Japanese penpal, and Pepe recounts his experiences in Japan. Even the "Word From Us Kids" focused on the country of Japan.
6. Fernlets by Fern: Fern's writing of haikus. (meh, this is sort of a minor one)
7. What Scared Sue Ellen? Sue Ellen makes a reference to the kappa, a demon in Japanese folklore.
8. The online world games on the Arthur website include these two countries.
9. Sue Ellen's trading card says that the place she would like to visit most is Japan (despite the fact that she's been all over the world).

References to Chinese and French cultures have also been quite popular (esp. with Emily's francophilia/French heritage, and the intruduction of Mei Lin to the show)

The show has also made minor references to other, relatively obscure cultures. This includes mentioning all of the countries in Africa ("In My Africa"), Sue Ellen preparing rendang, an Indonesian stew in What's Cooking? (it's quite good if you haven't tried it), Francine's family being Jews from Poland, and Mr. Molina's methods for staying cool during a heatwave and summer activities when he lived in Ecuador.

Finally, some countries and cultures are barely mentioned/not mentioned at all; the only time I can think of Germany being referenced is with Albert Einstein (counting Helmut the glasses designer may be a stretch). The Middle East hasn't really been touched except for the mention of historic Mesopotamia.
Vietnam and Korea have VERY RARELY been brought up in the series (the only things I can think of are Big Brother Binky, where, on the airplane, Binky can be seen reading a magazine with Vietnam on the cover, and the fact that Sue Ellen does Tae Kwon Do). In the community/city where I live, there are significant populations of Koreans and Vietnamese (in my school yearbooks, there would often be an entire row or more of Nguyens for each grade), and Korean culture is becoming ubiquitous in America, from the ever-so-popular Gangnam Style dance craze and the growing popularity of K-pop in mainstream music, to the various Korean BBQ places you see in many cities, so I thought it strange that shows like Arthur actually don't touch on Korean and Vietnamese culture very much (though it would be funny if Arthur had an episode about a parody of Gangnam Style taking over the internet and the radio).

Any thoughts on the multicultural aspects of the show, including what I've addressed here in this tediously long post ?

I too have noticed a lot of references to Turkey, Japan, China, and France, especially France. Not just because of Emily, but because I've noticed they incorporate the Eiffel Tower a whole lot, about as often as Albert Einstein, the only references to Germany that stand out to me. Kind of like how on The Dick Van Dyke Show, Rob, Buddy, and Sally the writers of The Alan Brady Show had used German professors too often in comedy skits.

Yeah, they do incorporate the Eiffel Tower a lot.

I've never watched The Dick Van Dyke Show, but that sounds pretty funny...like they're playing into a stereotype of some sort.

Anyway, as I've mentioned, the show makes quite a few references to the country and culture of Turkey as well, even outside of Adil's life...now, I don't happen to have a lot of Turkish friends, but many of my friends are Vietnamese, Indian, Korean, and Filipino. So while, in my community, at least, those four ethnicities/cultures greatly outnumber people of Turkish descent, I am surprised that they mention/show Turkey and its culture more often than they do those four countries/cultures combined. Perhaps they want to emphasize more obscure or less common cultures in the show?

I've also noticed that Mexico and its culture isn't mentioned very often in the series...surprising, considering how much Mexico is in the news and mainstream pop culture, and its being one of the countries that many people in the US are familiar with. Though Buster did visit Guanajuato, Mexico in PfB once.