I'll take a shot at this.
Assyria: The icon is very clearly the Assyrian Star, a long-standing symbol used as an emblem for modern Assyrian people. It seems to be derived from a symbol representing
Shamash, the local sun deity of Mesopotamia. The colors are likely simply representing the colors of the vast deserts the Assyrians settled in.
Brazil: As stated above, Brazil's symbol is derived from the
first Coat of Arms, the armillary sphere (representing the nation's history as a colonial entity) layered upon the cross of Christ (representing strong religious beliefs of the people). These were commonly displayed on a green field, though why the actual symbol is a darker green instead of a gold is uncertain.
Indonesia: It appears that it is likely to be the
Surya Majapahit, a Hindu symbol used extensively at the time of the Majapahit Empire, of course, which the game more closely takes cues from. Color choice is uncertain, it may simply be in reference to the blue of the Southeast Asian seas, and the dark red used as a symbol for the abundant spices of the islands.
Morocco: This is one of the more clear-cut ones: this symbol is the modern
Flag of Morocco. The color red has long been used to represent the nation, with the green pentagram a modern addition to signify the country's religious heritage.
Poland: This emblem is taken from the
Polish Coat of Arms, entitled the White Eagle, first established in the 13th century by the legendary Lech, founder of Poland. As Austria currently has the color scheme used for this symbol, the eagle here was changed to a maroon color.
Portugal: Once again, this symbol is from the
Portuguese Coat of Arms, depicting the five blue escutcheons in the center, the shape itself used to represent coats of arms, making it a symbol of regency and nobility.
Shoshone: Symbol is a standard headdress used by many Native American peoples, including the Shoshone tribes. Color connection is uncertain, may be meant to represent the dark brown of the soil and the bright blue of the sky, representing the great importance of nature to the peoples of this region.
Venice: As indeed pointed out above, this is likely the
Lion of Saint Mark, a Christian emblem of a revered saint, whose icon was frequently used as a symbol for Venice, both during its time as an independent republic and in modern flags and other cultural events. It is unclear what relevance the purple and off-white have to Venice, though it likely simply represents the frequent association of purple with wealth and power in Mediterranean cultures.
Zulu: As one can determine from Shaka's leaderscreen, this symbol is a traditional
Zulu cowhide shield, with the brown and light pink patterns cattle often possessed. Cattle were revered and seen as a sign of nobility in Zulu culture, and shields made from their hide are very much an emblem of prestige and militaristic prowess.
Gratefully these are all pretty simple and iconic emblems, unlike a few other earlier ones.