I nominate Edmond Hamilton's Interstellar Patrol series, starting with the novelette "The Star-Stealers" in Weird Tales February 1929 (available at the Internet Archive). Sorry I can't find a spaceport bar scene, but here's the next best thing, a galactic parliament. This is on a planet of Canopus, capital of the Federation of Stars:
Down we sped toward the mighty structure's base, down over the great buildings on either side which housed the different departments of the Galaxy's government, down until our ship had come smoothly to rest on the ground a hundred feet from the tower itself. Then the ship's hull-door was clanging open, and a moment later I had stepped onto the ground outside and was striding across the smooth sward toward the mighty tower. Through its high-arched doorway I passed, and down the tremendous corridor inside toward the huge doors at the end, which automatically slid smoothly sidewise as I approached. The next moment I had passed through them and stood in the Hall of the Council itself.
Involuntarily, as always, I paused on entering, so breathtaking was the immensity of the place. A single vast circular room, with a diameter of near two thousand feet, it covered almost all the mighty tower's first floor. From the edge of the great circle the room's floor sloped gently down toward its center, like a vast shallow bowl, and at the center stood the small black platform of the Council Chief. Out from that platform back clear to the great room's towering walls were ranged the countless rows of seats, just filling now with the great Council's thousands of members.
Beings there were among those thousands from every peopled sun in all the Galaxy's hosts, drawn here like myself each to represent his star in this great Council which ruled our universe. Creatures there were utterly weird and alien in appearance, natives of the whirling worlds of the Galaxy's farthest stars—creatures from Aldebaran, turtle-men of the amphibian races of that star; fur-covered and slow-moving beings from the planets of dying Betelgeuse; great octopus-creatures from mighty Vega; invertebrate insect-men from the races of Procyon; strange, dark-winged bat-folk from the weird worlds of Deneb; these, and a thousand others, were gathered in that vast assemblage, forms utterly different from each other physically, but able to mix and understand each other on the common plane of intelligence.
