Russia: Privet!

Antarctica: Hello!

Hawaii: Aloha! 

Spain: Hola, hermosa! 

Italy: Ciao!

Germany: Guten Tag!

Japan: Kon'nichiwa!

China: Ni Hao!

France: Bonjour/Salut

America: YO!

Canada: Hello!

England: Greetings!

Poland: So, totally, like, cześć!

Estonia: Tere!

Lithuania: S-Sveiki!

Latvia: Sveiki!

Korea: annyeonghaseyo! (I don't know, I used Google Translate xD)

Netherlands: Hallo!

Denmark: HEJ!!!!!!!!!!!!!!

Turkey: Merhaba!

Greece: Geia.

Switzerland: Gruezi.

Finland: Hei!

Iceland: Hello

Norway: Hallo.

Sweden: Jabba the hutt... Oh. Hej!

Romania: Alo!!

Bulgaria: Zdraveĭte

Scotland: Hello!

Egypt: ahlan wa sahlan

That's all the countries I can think of.... :3 Tell me in the comments if I missed some ^^