I'm not aware of many JP groups releasing their tracks in Korean. JP band BiSH would be great at Glastonbury. Blackpink, Japan's Band-Maid and other groups have had international success off the back of YouTube. They would do well pretty much anywhere, as other KR and JP bands have done, with no English lyrics required (although it's nice that they are acknowledging foreign fans).
Twice have always had a particularly strong JP focus. JP CDs have fewer extras but can come with DVDs. Korean ones usually come with substantial photo books and assorted extras. I usually get the Korean releases of Twice's CDs rather than the JP ones.
You don't need to understand Korean to enjoy their music, although the popularity of Kpop and Kdrama has seen a rise in Westerners learning the language. How many Americans understand Korean language? Ironically, SNSD wasn't SM's A-team, but B-team, but SNSD's success shifted the formula upside down. SNSD in this context means 7~9 member team consisting of each agency's best trainees(A-team). HIVE(aka BTS agency) is launching a female version of BTS produced by the SNSD producer who jumped ship because SM no longer had the ability to produce female teams(SM is basically like Johnny's now focusing on male idols only), but it would be in common interest of K-pop industry for someone else to succeed before HIVE's SNSD and YG's SNSD(aka 9 member Black Pink) launch in 2022. All prior attempts failed including SNSD(aka Girls' Generation)'s US launch which shocked the K-pop industry as SNSD is considered the greatest female idol team of all time in Korea and none comes close to them, not even Twice and Black Pink. I do wonder how this group will fair in the U.S. Jesus, are they old enough to do anything legal ?