As a Pole I can tell you this, 17 days ago (August 1st) there was anniversary of Warsaw Uprising (1944). I simply cannot imagine a mainstream Polish singer making a social media message in German or recommending a German song on this day. They would be blasted with hell fire by general public and pretty safe bet, also their own fans.
Regarding idols: How hard is to set up a notification in phone to remind oneself to not write in Japanese and/or share anything of Japanese culture/entertainment on this special day? These idols are super dumb either being literally uneducated of own history or just too lazy to simply pay attention to real life outside kpop bubble. A little effort would spare Korean people pain/anger and themselves a trouble of having controversy and having to make apologies.