How to translate "给力" in English?
It means awesome. All English translations for 给力 at mdbg.net: cool, nifty, awesome, impressive, to put in extra effort
http://www.mdbg.net/chindict/chindict.php?page=worddict&amp;wdrst=0&amp;wdqb=geili

The word has also found it's way into English slang in the form of geilivable 
http://www.urbandictionary.com/define.php?term=geilivable

Geilivable is not very common in western countries though, as far as I know.