yes...no...err...
What you see ingame is not mentioned here.
All what begins with TXT_KEY_ is somewhere mentioned in one of the files in the XML\Text folder.
For example, the america description is in Civilization IV\Assets\XML\Text\Civ4GameTextInfos_objects.xml (note: Not in the BtS folder, in the normal civ folder).
When you search in this file for "america", you'll get this:
PHP:
<TEXT>
<Tag>TXT_KEY_CIV_AMERICA_DESC</Tag>
<English>
<Text>American Empire</Text>
<Gender>Male</Gender>
<Plural>0</Plural>
</English>
<French>
<Text>Empire américain</Text>
<Gender>Male</Gender>
<Plural>0</Plural>
</French>
<German>
<Text>Amerikanisches Reich:Amerikanische Reich:Amerikanischen Reich</Text>
<Gender>Neuter:Neuter:Neuter</Gender>
<Plural>0:0:0</Plural>
</German>
<Italian>
<Text>Impero Americano</Text>
<Gender>Male</Gender>
<Plural>0</Plural>
</Italian>
<Spanish>
<Text>Imperio Estadounidense</Text>
<Gender>Male</Gender>
<Plural>0</Plural>
</Spanish>
</TEXT>
Here you can see, how america is named in the different languages.
If you just want to rename it, then you have to change it here in your language.
For example, just replace "American Empire" with "United States Of America", and it's done.
Edit: Sorry, i guess you'll have to change the spanish entry. (or better all, for homogenity)