Uh... Russian is my mother tongue! So... it depends in what situation you say this! If you say "Welcome" when somebody enters somewhere (a house, a country etc) then you say "Dobro pozgalovat" as they've already told here. But if you say "you're welcome" after "thank you" then it's compeletely different. It will be "Ne za chto" (word to word translation: that's for nothing) or which is probably better "Pozgaluista" :) And "thank you" will be "Spasibo" ;)!