The short answer
Korea and Japan differ in social platform usage. Rather than cloning the same content, design to each market's leading platforms and content culture.
In Japan, X (Twitter) is core to real-time discovery and Instagram is strong. Korea skews to Instagram, YouTube, and TikTok with faster trend response.
Messengers split into LINE in Japan and KakaoTalk in Korea, both central to retention and CRM. Design discovery and retention channels separately per market.
Localize channel mix and content format, not just language. Matching each platform's format and tone — even within one campaign — drives results.