It's an almost automatic response when you encounter someone who is getting ready to take a test, perform a song, give a speech or attempt to complete three phases of Japanese Embroidery in order to graduate by October.
Wishing someone luck implies bestowing upon them all good forces that can be mustered from the cosmos to support them in their journey. As if luck is just sprinkled down upon us like manna from heaven and success resulting from luck is accidental or the result of divine providence.
Conversely, the standard phrase used in Japanese culture when someone is facing a challenge has nothing to do with luck.
For the Japanese, the standard no-thinking-about-it reply is
ganbatte! or 頑張って！or がんばって
Gonbatte comes from the verb form gonbaru (頑張る, がんばる) which means to strive, to try one's best.
In other words, luck only comes through effort and working hard.
At the end of embroidery class in Japan, my sensei would say: Gonbatte kudasai!
And the students would answer: gonbarimasu (がんばります) or "I will work hard."
Today, I'm happy to take the luck of the Irish if it's being offered.
But I'll count on the wisdom of the Japanese and just plain get to work.
I've got class this Saturday and I'm trying to finish as much of the gold honeysuckle vine that I can by then.
Go n-éirí an t-ádh leat and gonbatte!