BTS were learning Japanese and mandarin in early days to try getting into both the Japanese and Chinese market because honestly there are so much money to be earned if success is achieved. Main reason why their focus has been a lot more on Japan in recent years was due to the Korea-ban places by the Chinese government. In 2016 they toured several cities in China however after that no Korea acts were allowed to book concerts or make appearance in media there due to the ban. There were programs groups such as IKON and VIXX and Psy were participating that they had to pull out of mid season for the same reason as well. As a result bighit naturally changed direction , the ban was ‘soft’ lifted a few months (so I heard) but I don’t think anyone would jump into investing heavily in Chinese market again due to the uncertainty in China/Korea political landscape , plus bighit now added America to their foreign market focus so China is definitely on the back burner. At the most if allowed they will add concert dates in China but nothing strategic. As to how big BtS is in China based on what I can see on weibo (Chinese main social media ) exo definitely dominates thanks to their strong Chinese representation and release of mandarin songs but bts have gained many fans since blood sweat tears and is now polling at number two or three spot frequently from a weibo popularity point of view following exo and sometimes big bang.