GOING GAGA: Gumpisms are definitely out. What's taken their place is Nellspeak. At the recent New York premiere of the Jodie Foster film, nearly every guest was trying to mimic the cabin-bound character. ''It's going to be everywhere,'' says Foster. But future Nellspeakers beware: The tongue (featuring ''ga'inja'' for guardian angel) is a tough one. Foster reveals that Irish-brogued costar Liam Neeson had the hardest time picking it up. ''We'd make jokes about it,'' says Foster. ''Liam would put this Irish thing on all the words.'' And Natasha Richardson Neeson's costar in the movie and in real life claims she also ''had a hard time. And I only had to learn a few words.''