While curious about the Centauri accent, I found this 2001 interview with Peter Jurasik (Londo Mollari) and Wortham Krimmer (Cartagia).

http://www.earth62.net/transcripts/jurasik22feb01.htm

The quick story about the accent, if I can tell you how I patchworked it together, is I was doing a play downtown, a Tennessee Williams play, and I worked really hard on a Memphis accent. I felt like I had really nailed it. But one L.A. critic nailed me and said, "That’s a terrible Memphis accent. That doesn’t sound like a Southern accent." I was really hurt. About that time was when "The Gathering," the pilot, showed up. I called Joe and said, "What do you want me to sound like?" He said, "Let him sound like whatever you want," so I purposely took a couple of different things. There’s a character who plays the parole officer in A Clockwork Orange, the guy who’s always saying, "And night-time is the best time, um, yes?" I took my Czechoslovakian grandmother. I had spent three consecutive summers in Ireland. I didn’t always take sounds; I took rhythms. Londo had a kind of musical thing.

The whole thing's worth a read, they seemed to be having fun.