With the release of Gods & Kings, 9 new civilizations and, of course, with them, symbols have been added. Of course, there also are a massive amount of new symbols with the many scenarios that have been released, so maybe we can investigate them. =)
Austria:
This is an odd selection for Austria, since it is actually the Bundesadler, the coat of arms used for the republic. The royal eagle of the
Austrian Hapsburg dynasty was two-headed, and held different items. The symbols on the republican eagle are the sickle for farmers, the hammer for workers, and a wall crown for the bourgeoisie. The icon used in Civ 5 misses the broken chains on the eagle's legs, which stand for freedom of the Nazi occupation after WW2.
Byzantium:
The symbol is the Byzantine or Orthodox cross, which is used as a symbol for the Orthodox Christianity which was founded in Byzantium during the Great Schism. The difference between the standard Christian cross and the Orthodox one is that the Orthodox cross has a slanted footrest, and a second cross bar.
Ethiopia:
The Lion of Judah is an important symbol for Ethiopia. Its importance is twofold. For once, it is a symbol directly representing Haile Selassie and his Rastafari movement. However, the symbol's ties with Ethiopia are much older, and generally represent the imperial dynasty of Ethiopia.
Sweden:
The three crowns, Tre Kronor in Swedish, have been a widely used emblem in Scandinavia since the 13th or 14th century, and then turned into the national symbol of Sweden after Gustav Vasa terminated Sweden's membership of the Kalmar Union.
The Netherlands:
Taken from the Dutch royal coat of arms, the symbol represents a crowned lion holding a sword and a bundle of 7 arrows. Fittingly, the coat of arms was introduced by William himself. However, lions as coats of arms have been used for a long time before the Netherlands were even founded - almost all of the city states and small duchies in the area were using lions of some sort in their coat of arms.
Celts:
A traditional Celtic flower design. Not sure where it comes from specifically, though.
And those where I don't know anything about...
Maya:
Huns:
Carthage:
Maybe a reference to Hannibal's crossing of the alps with elephants?
As for the Scenarios...
Anglo-Saxon England:
This might be a reference to some early cross design? I don't know where it comes from, though.
Normandy:
Quite a traditional symbol, the coat of arms of Normandy has been 2 golden leopards on red background for a long time. Occasionally 3 leopards are used (such as in the coat of arms of the United Kingdom), however, the 2 cats are the more common ones.
Norway:
Another traditional coat of arms, Norway's lion holding an axe - which is named den norske løve in Norse. Apparently the prototypes for this symbol date back to the 11th century.
Almohads:
This is based on the flag of said dynasty, which is a chess board on a red flag. The symbol version is simply stylized.
Ayyubids:
An alteration of Arabia's symbol in the standard game. The Ayyubids used a purely golden flag for their realm - the crescent moon and star in the middle have the same color. But since all civilizations need 2 colors, I guess the lighter yellow was added.
Hittites:
Based on a carving found in a former Hittite temple in Alaca Höyük, Turkey.
Sumer:
Lamassu, a Sumerian deity.
Manchuria:
Based on the Qing dynasty's flag, a blue dragon with a pearl on golden background. The rest of the dragon's body was removed.
Goths:
Based on Visigoth eagle brooches.
Franks:
A stylized version of the Carolingian cross.
Vandals:
No idea.
The symbols for Samoa, Tonga and Tahiti in the Polynesian scenario are simply variations of Polynesia's Koru. And of course the Empires of the Smoky Skies symbols are completely fictional.