つまらない の訳語→ bad bleak boring disappoint disgusting fool foolish fucking futile goddam horrible inadequate intolerable menial pedantic peewee prosaic rubbishy tedious unexciting uninteresting wearisome wretched zilch
わかりにくい の訳語→ confusing convoluted hacky
退屈な の訳語→ enduring irksome poor prasaic tiring unlively
読みにくい の訳語→ illegible indecipherable
判読できない の訳語→ undecipherable