I've traveled to a lot of countries so I'll write what I've experienced. Of course, most of these trips were only for a week or so, which isn't really enough time to gauge an entire culture, so if you live in any of these places please correct me if I'm wrong.
America: High waisted pants have made a comeback, making whaletails very improbable. However, I think more girls wear thongs than people realize. In high school, I saw a lot of whaletails with jeans and even see through dresses and skirts revealing thongs (we all know that if a girl is willing to wear a thong under a dress they probably wear thongs a lot). America is huge, so it depends on where you are. I'd say the more conservative areas (south, midwest) might be more restrictive of thong wearing, but more liberal parts can also be. Conservative areas have that slut shaming and name calling kind of aspect at times, but liberal areas also tend to shame whaletails a bit. However, with the trend of tolerance and acceptance, I feel like showing off thongs are a bit more accepted in liberal parts (west, north-east). They're also more popular in teens to 30-40 year olds I'd say, mostly because a lot of older women grew up in an age where thongs were seen as slutty and were looked down on. But after the 2000s, where girls wore thongs all the time, most of those women are in their late 20s, 30s, maybe almost 40s, but above that I'd say it decreases a lot.
Netherlands: (In Amsterdam at least) I only spent a week there but saw tons of thongs every day. Whaletails, see-through pants, see-through dresses, you name it. Any way you can see a thong you can probably see in Amsterdam. One girl was even wearing a completely sheer black dress, showing off her matching black bra and black thong. Of course, Amsterdam attracts a lot of tourists, so a lot of these women could be tourists, but still, Amsterdam has a very relaxed atmosphere, if you're thong is showing no one seems to care, people just live their lives.
France: France has a very conservative clothing style, so a lot of high waisted pants and nearly no tight pants, so pretty much no yoga pants, leggings, leather, or anything. A lot of very loose, high waisted clothing, making it hard to spot whaletails, so I'm not sure how many girls actually wear thongs. I spent nearly 2 weeks in France this summer and didn't see a single thong slip or anything. Considering that their style is more conservative, I would assume they don't wear thongs too much, but I'm not sure, maybe a French person could answer better.
Germany: Kind of an in-between of Amsterdam and France. They definitely have a bit more of a relaxed atmosphere like Amsterdam, but I didn't really see many slips like France. I did see a good amount of see-through leggings with thongs underneath, and even saw a girl out on her balcony in the morning in her bra and thong. I get the feeling most German girls wear thongs, mostly because they give me a 2000s (American) girl kind of vibe, which means a lot of thong wearing and thong showing. But I'd say Germany's kind of an in between from Netherlands and France, leaning more towards Netherlands.
Belgium: I mostly stayed in the northern part of Belgium, where they share more similarities with Amsterdam, rather than the south, which is more French/German (I think). But in the north, it was pretty similar to Amsterdam, saw some thongs, I think their attitude is probably the same as the Netherlands. Can't really say about the south, but I'd assume it's more like Germany/France.
Canada: I'd say pretty much the same as America, except for Quebec. Quebec is more like France, so a bit more conservative and might look down on thongs a bit more than in the rest of Canada, but I don't think it's too extreme, so it's safe to say they still wear thongs a lot.
Italy: They also wear a lot of flowier, looser clothing like in France, however when I went to Italy I saw multiple white thongs through white rompers/dresses, and a few more thongs through pants. The good thing about Italy is that they seem to like brighter clothes, meaning a lot of white, meaning it's easier for the clothing to be see through, especially if they wear a black thong. This is also true in France, but I didn't see as many see-through rompers in France, unlike Italy, where I saw quite a few. I'd say Italian girls wear thongs pretty frequently. Since France and Italy have loosely similar clothing styles, I would hope this means French girls wear thongs a lot too, but I'm not sure. Italy has a pretty friendly atmosphere, so I don't think they'd be upset about seeing a thong in public.
Britain: A bunch of thongs. Similar to Amsterdam, I saw quite a few everyday. However, most of them were in London, which leads me to believe a lot of them were tourists, like in Amsterdam. But even in some smaller parts of Britain, I saw some see-through leggings. In London, though, I don't really recall seeing many whaletails, but a ton of see-through leggings, probably because it gets pretty cold over there. I'd say it's pretty accepted to wear thongs there because a lot of the girls wear leggings or yoga pants since it's cold a lot of the time there, and most girls wear thongs under leggings or yoga pants.
Sweden/Denmark/Norway: I was really young when I went, and didn't even know what a thong was, but I do remember seeing a whaletail and wondering what it was. Now, I think most Scandinavian girls wear thongs, as evident by the sportswomen page on this site (lots of Norwegian girls wearing thongs) and these 3 countries share very similar cultures, so I'd say it's safe to assume they all have similar attitudes on wearing thongs, which is a lot.
Asia: I have absolutely no idea about the less developed Asian countries, but I'd say next to no thongs. In the more developed countries in the east, (China, Japan, Korea), I'd say it's a bit more frequent, but still uncommon. I'm half Chinese, and they are pretty restrictive about thongs and wearing revealing clothing. Thongs are seen as too sexual, something a lot of Asians family wish to avoid around teenagers, and would probably earn a shaming by the family if caught wearing them. However, I think once girls move out, they may wear thongs every once in a while. K-pop stars and other Asian stars seem to wear thongs, probably because they're more westernized. But yeah, it's pretty uncommon. Even if it was more common, the only way to find out is to be from there or live there and find out for yourself, because I highly doubt you'll be seeing a single thong slip anywhere from a local.
Mexico/Latin America: Didn't see any thongs whatsoever, saw a lot of panty lines through shorts/pants. However, they do have a party side, so I'm sure they turn to thongs in that case. Can't say about Latin America, but I'd say it's slightly similar, maybe with more thong wearing. It always depends on where you are, though. Some areas of Mexico are poorer and I doubt they wear thongs, but if you get in the heart of a big city in Mexico or Brazil and go to a club, you'll see a ton of thongs.
Again, this is just what I've observed from a few short trips, I've only lived in America and have family from China, so those are the only cultures I can truly speak on, the others are just observations, feel free to correct me.
