Hi, Hello, How Ya Doing?, What's Up?, Hey, Howdy, Yo...

1. Icelandic - góðan dag (formal; pronounced gothan dahg), hæ (informal; pronounced "hai")

2. Lithuanian - laba diena (formal), labas, sveikas (informal; when speaking to a male), sveika (informal; when speaking to a female), sveiki (informal; when speaking to more than one person).

3. Punjabi - sat sri akal

4. Swahili - jambo, Habari (hello), Habari gani (How are you?)

5. Thai - sawa dee-ka (said by a female), sawa dee-krap (said by a male)

6. Welsh - shwmae (South Wales; pronounced shoe-my), "Sut Mae" North Wales( pron "sit my") or "S'mae" ( Pron "S' my") or simply "Hylo"

7. Yiddish - sholem aleikhem (literally "may peace be unto you"), borokhim aboyem or gut morgn (morning), gutn ovnt (evening), gutn tog (day), gut shabbos (only used on the Sabbath)

8. Turkish - merhaba selam (formal), selam (Informal)

9. Greek - γεια σου (pronounced yah-soo; informal), γεια σας (formal)

10. Bengali — namaskar (In West Bengal, India)

Just for fun:
Ung Tongue - Hello (This is a made-up language, like Pig latin. This is pronounced Hung-ee-lung-lung-oh.)

Klingon - nuqneH? [nook-neck] (literally: "what do you want?")

(http://www.wikihow.com/Say-Hello-in-Different-Languages)
(http://en.wikipedia.org/wiki/Bengali_language)

Now, I just need to start memorizing them. I will only memorize one or two general greetings per language, instead of focusing on morning, afternoon, and evening greetings. Grad school is taking up a huge portion of my memory space.