While NEM & MONA are interesting cryptocurrency, i wouldn't invest my money on any cryptocurrency which only focus on specific country.
But, i'm sure once majority users adopt SegWit and use LN, i think Japan crypto community will consider accept BTC beside NEM & MONA.
Yes of course, BTC will be accepted as long as fees drop
Right now the fee stands at 510 Sat/Byte, which is lower than what we had two weeks ago (1,000+ Sat/Byte). But even now, close to $50 is needed as transaction fee for an average transaction and the situation is not very viable. Perhaps that is one of the reasons why the dominance of BTC has dropped to 38% from the level of 61% which we had a month before.
In Japan, you can already pay in some stores with BTC, but MONA is developing, and several cryptos can co exist it is not a problem. Also, Japan is leading technology development in the Asia Pacific area, not to say worldwide so I would not say it is focused on one country!