You could say, "Thank you" or "Thanks". If you will never see the person again, you could say, "Good Luck to you as well", which means good luck in life, and not good luck in your travels. :DNo.1
Could you please tell me when people say 'Good luck' instead of 'Goodbye' what should I say? Is it 'Good luck' or 'Goodbye'?
Both: Thank you. Goodbye. :D If they too are traveling, you could reply, "You, too", meaning I, too, wish that you, too, will have a pleasant journey.No.2
When seeing somebody off people would say ' I wish you a pleasant journey'. What should I say? Is it 'Goodbye' or 'Thank you'?

Student or Learner
Bookmarks