Well, if you're using vanilla, follow this filepath:
C:\Program Files\Steam\steamapps\common\Sid Meier's Civilization V\Assets\Gameplay\XML\NewText
It will take you to a folder that includes the XML file that defines "English" for the game, credits, and another folder, "EN_US", which contains all the game text in XML files of different categories, like city names or technologies. Sadly, it seems, vanilla only includes English.
The equivalent filepaths for G&K and BNW go more like this:
G&K: C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\DLC\Expansion\Gameplay\XML\Text
BNW: C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\DLC\Expansion2\Gameplay\XML\Text
They both have folders and credits for German, English, Spanish, French, Italian, Japanese, Polish, Russian and Chinese in that order, so pick your favorite.