Figaro said:
There are thousands and thousands of different languages in the world; the only one I can think of that is shared by more than one Civ4 nation is English (England and America). Even in most scenarios I don't think it would affect much.
Yes...ish. You're looking at how the world looks now. But what if you check the entire history of some of the civs? Some cultures adopted / were forced to adopt foreign languages. The Roman Empire is a great example, since it forced its language upon others (some of which are civilization IV nations, eg Egypt) and partly adopted a foreign language itself (Greek).
Some other examples what can happen to languages after a clash of power:
A language evolves differently in separate regions/populations, until there is no longer mutual intelligibility:
Latin --> Spanish, Italian, French
(In Civ IV terms: If two nations with the same language are at war or stop all trade relations, their languages will gradually split up.
A population is subdued by another population and partly retains the language. Through the co-existence of both languages, the 'subdued' language evolves.
Old English + French --> Middle English
(In Civ IV terms: If you capture a city, its language will gradually become more similar to your official language. Depending on how soon the city is captured back by the original owner, the city's language may have become unintelligible to his empire.
A language is completely and successfully suppressed by the dominating power. The dominating language does not adopt features of the suppressed one:
Native American languages, Celtic languages in large parts of Britain
(In Civ IV terms: If you decide to suppress a language, it may become extinguished over time. This leads to unhappiness during the process, but once it is extinguished, the city will lose an important link to their former nation)
On the level of Governments, there is the question as to how to react to language differences. Shall there be several official languages (Switzerland, India) or just one? May the people retain their native language and learn an official language in addition or do you want to suppress any other languages than the official one(s)?
These questions not only have an impact on diplomacy, but also on happiness and productivity.