Thanks for taking the time to report this!

As these strings are not in http://git.wikimedia.org/blob/mediawiki%2Fextensions%2FEcho.git/HEAD/Echo.i18n.php they are defined in MediaWiki core in the file

languages/messages/MessagesKo.php:

'minutes-ago' => '$1분 전',

To compare,
languages/messages/MessagesEn.php:

'minutes-ago'  => '$1 {{PLURAL:$1|minute|minutes}} ago',

Moving to MediaWiki -> Internationalization.

kht1992 wrote:

but why are they shown in English if it is plural? I guess that {{PLURAL:}} function is not working properly. Now I've just changed translation which will not effect in PLURAL function, but I'm just worrying about other messages using PLURAL function.

(In reply to comment #2)

I'm just worrying about other messages using PLURAL function.

Not sure if I understand correctly, as the Korean translation did NOT use the PLURAL: function.

kht1992 wrote:

(In reply to comment #3)

(In reply to comment #2)

I'm just worrying about other messages using PLURAL function.

Not sure if I understand correctly, as the Korean translation did NOT use the
PLURAL: function.

What I'm saying is not the fact that Korean language do/do not use plural form when they count the date. The point is, MediaWiki:Minutes-ago translation was "$1{{PLURAL:$1|분}} 전" and it caused an error. Do we also have to initialise plural forms like "$1{{PLURAL:$1|분|분}} 전" although there's no plural form in a language?

I still don't understand what is the error. "$1분 전" is OK as translation in technical sense. You don't need to use the plural magic word in Korean.

kht1992 wrote:

(In reply to comment #5)

I still don't understand what is the error. "$1분 전" is OK as translation in
technical sense. You don't need to use the plural magic word in Korean.

It will not cause any error anymore, but at the time when error caused, the translation was like "$1{{PLURAL:$1|분}} 전". Isn't it just same as "$1분 전"?

kht1992 wrote:

And also, there's no "Mediawiki:Days-ago", but why they causes an error?

(In reply to comment #4)

The point is, MediaWiki:Minutes-ago translation was
"$1{{PLURAL:$1|분}} 전" and it caused an error.

So you're saying that this "fixed" it? https://translatewiki.net/w/i.php?title=MediaWiki:Minutes-ago/ko&diff=5020271&oldid=4291555
Could you please clarify the steps to reproduce? If I understand correctly, they should be:
0) Pick a wiki with Echo where you're sysop, change [[MediaWiki:Minutes-ago/ko]] and friends to "$1{{PLURAL:$1|분}} 전" etc.

1. Write on your own talkpage from another account, e.g. logged out.
2. Wait 1 second, minute, hour, day or month (but not 2)
3. Visit Special:Notifications?uselang=ko

I can reproduce it with 1 month, see attachment.

(In reply to comment #7)

And also, there's no "Mediawiki:Days-ago", but why they causes an error?

No idea. There are several "ago" messages, including some pin CentralAuth and Flow (which shouldn't be the problem here) and of course the infamous [[MediaWiki:Ago]] introduced by Werdna 2 years ago amidst the general revolt, for the status of which we never got a clarification, so it's hard to tell. I also no idea which message is at fault here (qqx doesn't tell) and I don't have time to check the above steps now, sorry.
https://translatewiki.net/wiki/Thread:Support/ago_%28%22$1_ago%22%29_cannot_work

Created attachment 13442
Special:Notifications?uselang=ko with "1 month ago"

Attached:

kht1992 wrote:

(In reply to comment #8)

(In reply to comment #4)

The point is, MediaWiki:Minutes-ago translation was
"$1{{PLURAL:$1|분}} 전" and it caused an error.

So you're saying that this "fixed" it?
https://translatewiki.net/w/i.php?title=MediaWiki:Minutes-ago/
ko&diff=5020271&oldid=4291555
Could you please clarify the steps to reproduce? If I understand correctly,
they should be:
0) Pick a wiki with Echo where you're sysop, change
[[MediaWiki:Minutes-ago/ko]] and friends to "$1{{PLURAL:$1|분}} 전" etc.

1. Write on your own talkpage from another account, e.g. logged out.
2. Wait 1 second, minute, hour, day or month (but not 2)
3. Visit Special:Notifications?uselang=ko

I can reproduce it with 1 month, see attachment.

(In reply to comment #7)

And also, there's no "Mediawiki:Days-ago", but why they causes an error?

No idea. There are several "ago" messages, including some pin CentralAuth and
Flow (which shouldn't be the problem here) and of course the infamous
[[MediaWiki:Ago]] introduced by Werdna 2 years ago amidst the general revolt,
for the status of which we never got a clarification, so it's hard to tell. I
also no idea which message is at fault here (qqx doesn't tell) and I don't
have
time to check the above steps now, sorry.
<https://translatewiki.net/wiki/Thread:Support/
ago_%28%22$1_ago%22%29_cannot_work>

Nope, it's still not fixed. Still presenting "1 minute ago" and "1 hour ago".

MediaWiki.org shows untranslated "1 month ago" for me.

We also have the same problem I moved but they do not work

https://translatewiki.net/w/i.php?title=MediaWiki:Centralauth-hours-ago/ce&action=edit

+i18n; +need-volunteer, +testme, +upstream per possible CLDR culprit (bug 57643 comment).

Is this because $timeUnits in extensions/cldr/CldrNamesK/CldrNamesKo.php ( https://git.wikimedia.org/blob/mediawiki%2Fextensions%2Fcldr.git/91eacf88522dc565b5a1f19e5f150465582db19d/CldrNames%2FCldrNamesKo.php#L1155 ) does not have any of "hour-past-few", "hour-past-one" only "other"?

Maybe "other" should be the fallback?

Hello I'm sorry that I write here but I would like to know how you solved this problem? we, too, these words were not translated if you got it I would have corrected in Korean us {{PLURAL: $ 1 | is inappropriately. Intentioned writing.

https://gerrit.wikimedia.org/r/116722

FYI, this also happens on the Flow too.