Topic on Project:Support desk

Is it able to override MediaWiki core interface messages with extension?

Yes, but it's a mess. The WikimediaMessages extension does that for Wikimedia.

How can I do that? I put messages to override on en.json and ko.json but it doesn't override messages that already exists.

You'll need to define the new key/values with a different key, and write some code hanging off the appropriate hook – e.g. https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/WikimediaMessages/+/master/WikimediaMessages.hooks.php#15 is how we intercept most keys, replacing foo with wikimedia-foo dynamically.