Can anyone tell me which season & episode that comes from?

I think its an early episode or a flashback one.

Dorothy says to Sophia something like "lets go out on the lenai", then Sophia asks "what's a lenai?" and Dorothy says something like "its a porch" then Sophia goes, "well excuse me Krystle Carrington!"

It's an episode in Season 2 called "A Piece of Cake." They are all reminiscing about past birthdays. The scene you are thinking of is when they are celebrating Blanche's birthday and Sophia comes to the house to celebrate. Everyone is out on the porch except for the girls. Sophia walks in and Dorothy tells her that the party is on the lanai. "What's a lanai?" "The porch." "Well excuse me Krystle Carrington." It's a really funny episode.
This is also the episode when they all remember the party for Dorothy that was at Mr. Ha Ha's Hot Dog Hacienda. It was hilarious. Poor Dorothy! :lol:
I hope that helps. ;)