This is what I have so far ... When you roll your mouse over someone elses territory but under the fog of war, you see something like "American Empire". The word EMPIRE appears in the following XML files (ignoring civilopedia) ...
...\Assets\XML\Text\CIV4DiplomacyText.xml
...\Assets\XML\Text\CIV4GameTextInfos_Objects.xml
...\Assets\XML\Text\CIV4GameText_Misc1.xml
The ones in diplomacy are all about "our empire hates you, we declare war".
The ones in GameText_Misc are "our empire grows" as well as scenarios.
The ones in GameTextInfo_Objects is like this ...
<TEXT>
<Tag>TXT_KEY_CIV_AMERICA_DESC</Tag>
<English>
<Text>American Empire</Text>
<Gender>Male</Gender>
<Plural>0</Plural>
</English>
So, the gametextinfo looks like the one.
I searched for EMPIRE in the python files and found one occurance in ...\Assets\Python\EntryPoints\CvTranslator.py This changes it into "player.getCivilizationDescription(iCase)".
I searched for 'getCivilizationDescription' in the python files and found the following references
...\Assets\Python\CvAdvisorUtils.py
...\Assets\Python\CvEventManager.py
...\Assets\Python\PyHelpers.py
...\Assets\Python\EntryPoints\CvTranslator.py
...\Assets\Python\pyWB\CvWBDesc.py
...\Assets\Python\Screens\CvDawnOfMan.py
...\Assets\Python\Screens\CvTopCivs.py
I'm now in the process of hunting through these files (sigh).