Yeah, I did that. Here's an example of a file I altered:
Again, that's completely wrong. If, for instance, TXT_KEY_CIV_AMERICA_ADJECTIVE already exists in the game database (which it does), then you CANNOT add a <Row> declaration that uses the same Tag. What you had there absolutely would not work.
A <Row> entry means "add a new line to the database, added to the core game's lists". An <Update> entry means "change one or more elements of an existing line in the database".
Either way, if there's an entry in the main database that you don't intend to change, then
it should never appear in your file, anywhere.
Go read Kael's beginner's guide a few times.
Could I still use "<Civilization_CityNames>" or would I have to rename it?
Okay, you're really confused on how XML works. XML table names, like Civilization_CityNames, are absolute. They're tied to a table declaration at the top of the original file. Any changes you make MUST use the same table name, unless you want to add an entirely new table declaration (and if you do that, then you also need to write the Lua code to actually do something with that table, since the core game won't look for any tables that didn't already exist).
If you're modifying the city names for an existing civ, then you'll need to use a lot of <Update> tags. If you're creating a list of names for a new civ, you'll need to use a lot of <Row> tags.