What does 여기 심심해 mean in Korean?

What is the difference between 별 말씀을 and 괜찮아? When are they used?

How do I say this is Korean? It's okay and welcome. I want it in one sentence as a reply to someone who apologised and thanked me.

What does 저는 셀테기 ㅠㅠ mean in Korean? This was posted on my selfie. Is it suppose to be funny? What do I respond to his?

How to say this in Korea "Yes, in Australia it is summer in December" I want this to be the standard formal Thank you.