I don't believe in monolithic aliens who all talk exactly alike,
with the same accents, any more than I believe in humans who all talk
exactly alike. Hence, the difference between Vir and Londo.

When they're together in their quarters, they're talking in Centauri,
which we hear as English, as in WW II movies, the Germans are obviously
talking to each other in German, but we hear it in English. (The only
other alternative is to subtitle whole lengthy segments of the show, which
is both unworkable, awkward, and unfair to blind viewers.)

jms