Lang_ZTF03 comes from ZooTek and I downloaded it in Dec '05. It will not overwrite ZTF01 but 01 should be removed to prevent any conflicts in the game. Here is what is posted in the download section at ZooTek:
TO INSTALL:
Extract Lang_ZTF03.dll to your Microsoft Games/Zoo Tycoon folder.
Delete any of the following .dll files from the Zoo Tycoon folder if they are present:
Lang_ZTF01.dll
Lang_ZTF02.dll
Lang-zootek.dll
Lang1stAid.dll
Lang-ztcastle.dll
The Complete Tunnels Pack is included in Lan_ZTF03.
One way to clarify where a give donwload came from is to setup a new folder somewhere outside the Tycoon game files and store the zip files there in a sub-folder for each website you download from. Mine looks like this:
Drive C:
Download
ZooTek
Zoo Admin
ZT Unleashed
Zoo Keepers Lounge
I also add a code to the name of the zip file when I save it - ie ZA_ , ZTek_ , ZTU_ - that way if a file gets moved, I know where it belongs.