add jp locale ("sort of" fixes ccbrown/gggtracker#108)

[derekmcq]

first time using Go, I ran the pre-commit successfully

the translations I've added are straight from google translate, I used "Author" instead of "Poster"

[ccbrown]

Thanks! I've made some suggestions on the translations, but otherwise I think this looks good! I'll see if I can find time to further test it and deploy it later this week.