Delenn mentions at the end of season 5 that English is the language of comerse and trade (meaning it's wide spread, at least in that sector)
Actually, Delenn called English Earth's
commercial language, not the common language of commerce and trade throughout the sector. The point being that Humans themselves don't have a truly "universal" language except by habit and convention. For that matter we know that the Minbari have several languages and many dialects, and I would assume this is also true of most of the alien species. I would think a world where everyone spoke the same
language would be a rarity.
OTOH all worlds probably would have one "common" langauge for international diplomacy and trade (as at various times and in various places Attic Greek, Latin, French and English have been for Earth) for the simple reason that it is more efficient for everyone to have a common second language than for everyone to learn all possible languages. It would be this dominant language that alien races would learn for interplanetary communication.
So English would be "the" language for Earth, the Religious caste dialect for the Minbari, one form of Narn "Narn", and the language of the Centauri court "Centauri". (Just as the court-influenced dialect of Paris became "the" French that dominates France and which is taught abroad.)
Because the Centauri conquered and occupied much of our corner of the galaxy at one time, and dominated a good deal of the rest, their language would be very widely spoken, and probably used for communication between many races. Since the Dilgar war English has become more and more important, while the Centauri influence has waned. Minbari was never widely spoken because the Minbari mostly colonized empty worlds with their own people, rather than conquering existing peoples. Even 1,000 years ago when they led the Armies of Light Centauri was probably more widely-spoken, and therefore would have remained the common language.
So I think most of the exchanges between aliens of different species aboard B5 would have been in English or Centauri, while those between members of the same species would be in one of their own native tongues.
BTW, I suspect that G'Kar spoke Centauri as little as possible, especially
to Londo. He would never have given him the satisfaction. He would have enjoyed making Mollari speak English instead. During their time on Centauri Prime this would have had the advantage of keeping their conversations more private, since I doubt that many members of the insular and arrogant Centauri court (as opposed to their diplomats) would have bothered to learn an off-world langauge like English.