As far as I know, historically the rank benchmark in Europe has been that the highest amateur level is 6d, while a few top amateurs of similar level as top insei or low ranking pros were considered 7d (as sort of an honorary rank). From there, other ranks are more or less determined by handicap.
I think this benchmark originally came from Japan, but Japanese ranks have since inflated somehow, so that in the early 90s some Japanese 6d were like 3d in Europe, while some other Japanese 6d were like 6d in Europe. When I was in Japan in 1990, I played as a 4d, while my Dutch rank would have been 1k-1d.
There are 2 Dutch players who are/were considered 7d in Europe: Ronald Schlemper and Rob van Zeijst, who have both been top insei in Japan and both of them were European champion 3 times.
Ronald Schlemper also took 3rd place in the WAGC in 1988, behind Japan and China.
In the late 80s and early 90s ranks in the Netherlands were about 3 ranks tougher than ranks in Japan (at least in the dan range).
At that time, there weren’t many Chinese amateurs playing in Europe, but the few there were did seem to have ranks that were similar as in the Netherlands, perhaps even a bit tougher (by about half a rank).
I don’t know much about lower ranked Korean amateurs at that time. In my memory Korean amateurs playing in the European Go Congress in the early 90s were all 7d and typically only lost to each other. The strongest Korean amateurs who are active in Europe today are ranked 8d EGF, overlapping with low-mid rank pros. Their level may be similar to the level of Baduk Doctor who is a Korean 7d.
From this it seems that at least Korean high dans are about 1 rank tougher than European high dans, and this hasn’t changed much over the last 3 decades.
I think the introduction of the European rating system around 2000 caused some deflation in Europe (at least in ratings, ranks to a lesser degree), but it was only on the order of a 0.5 rank deflation in the dan range over a period of 20 years.
In 2019-2021 I was involved in updating the European rating system, repairing this deflation by (among other things) inflating EGF ratings by about half a rank on average (in the low-mid dan range).
In the early 90s, I think AGA ranks were considered about 2 ranks softer than Dutch ranks, so AGA ranks would be more or less in between Dutch ranks and Japanese ranks. But I think that increased (online) contact between European and American players on KGS in the early 2000 reduced this rank gap. Nowadays the gap between AGA ranks and EGF ranks seems to be only about 1 rank around 1d level, though possibly still 2 ranks at mid-high dan level. A possible explanation for the latter is that KGS high dans were inflated compared to EGF high dan levels. In the 2000s it was not uncommon for 6d EGF to have a 9d+ KGS rank.
I’m not aware of the rank gap between Japan and Europe changing significantly since the 90s, so I’d assume that Japanese ranks are still about 3 ranks softer than European ranks (at least in the 1d-5d range).
In China a lot seems to have changed in recent years. Chinese dan ranks below 6d seem to be all over the place nowadays, especially for younger players.
My impression is that nowadays stronger Chinese 5d are similar to 5d EGF (perhaps even 6d EGF), while weaker Chinese 5d are more similar to 1d EGF.
Currently there is a high dan player from China active in the Netherlands. Given his rank history in the EGD, I assume he was 5d in China, but he is more like 6d in the Netherlands: https://www.europeangodatabase.eu/EGD/Player_Card.php?&key=19498325
As for Chinese kyu ranks, I know some cases of kids in Europe who would be about 15k here (as in needing about 9 handicap against a 6k EGF), who had their ranks estimated by their online Chinese teacher, and the estimate of the Chinese teacher was more in the range of 10k, possibly as high as 5k.
I also know of 2 Taiwanese amateurs playing in the Netherlands. From them I’d think that Taiwanese dan ranks are about 2 ranks softer than Dutch dan ranks. One was 6d in Taiwan and he is 4d in the Netherlands, the other was 1d in Taiwan and she is 2k in the Netherlands.