Answer 1

There’s really no such thing as the most useable program anywhere because each programming language has its own strengths and weaknesses. However, here’s some things on the most popular languages in Japan: https://qph.ec.quoracdn.net/main-qimg-4a660cee8fbd1b337192b1a70c8d7e2e
https://www.quora.com/What-programming-languages-are-popular-in-Japan
http://lang-8.com/topics/6304
https://www.japan-guide.com/forum/quereadisplay.html?0+149886

Hope that helps!