I have put a tweak into the policy on names to include "and major cities in Korea". I wouldn't object to a few more (we have 4 in the south and one in the north.) I'll close this now.

The funny/curious thing about Korean placenames is that even though they are read as if they were written in katakana, they are 
actually often written in kanji. The big exception is ソウル, which is never seen in kanji, because it is a word that is based on 
pure Korean rather than Sino-Korean. Virtually all other placenames in both South and North Korea are Sino-Korean.
An example of an article where "Daegu" was written as 大邱: https://book.asahi.com/article/13370476
An example of an article that uses kanji with katakana pronunciation between brackets (perhaps the most common way to deal with 
things): https://news.yahoo.co.jp/articles/a5751db09004a188d2de8db8ce0729d386681ae8

I do understand the logic behind e.g. American placenames being more frequent in Japanese media than Korean ones, but perhaps Korea 
should not be treated the same as e.g. Spain. I wouldn't say more placenames have to be added for every country, but maybe at least 
the other countries in the Sinosphere (i.e. Korea and China) deserve higher prevalence.

https://www.edrdg.org/wiki/index.php/Editorial_policy#Proper_Names

It's been a bit of a balancing act to come up with in/out rules for place names. We need to be consistent, even-handed, etc. as well as being actually useful. It's been a while since I read the Japanese press much, but I got the impression that city names in the US and Europe occurred more often that cities in Korea, and I think the Korean ones were often in katakana (could be wrong).
I wouldn't object to some more Korean cities being included in addition to the four we have now, but I'd prefer that we started from a recognized policy and implemented it. As I commented in another entry, we'd allow Seville and Barcelona in Spain, but probably not Granada. That's the sort of criteria we're using with Korea.

It's a Japanese-English dictionary, so Japanese and English names take priority.

May I ask why, for example, "states and provinces of English-speaking countries and their capital cities" are seen as appropriate 
dictionary entries, while names of major cities in the Sinosphere are not? I know that the Korean place names I added already 
appear as part of the name dictionary, but the point of adding them to jmdict is that one would in one glimpse be able to see four 
different things, namely: (1.) the placename in kanji; (2.) the placename in katakana; (3.) the placename according to on-yomi 
(which in case of Korea is archaic but not unuseful information); and (4.) the English (or alphabet) notation. If you ask me, this 
is a lot more useful information compared to the placenames of English-speaking countries, which only mention (2.) and (4.). But 
again, that's just me. I would love to see your opinions.