The following list was updated January 7, 2021 by Jay.
Here is a list of lang ".dll" files for Zoo Tycoon that contain at least 1 message that is not in any other ".dll" file. Those starting with lang0, lang1, lang2, or lang3 are official ones that can be found on either the game's CD-ROMs or via download. Some of the user created ".dll" files might be currently unavailable. It is unlikely langZTC.dll will have further additions, but it is listed because it is a combined ".dll" for a web site. langAztecThemePack_engl.dll contains English messages. There is also a langAztekenThemePack.dll that contains German messages. Those two were available at the Zoo Tycoon ABC site.
lang0.dll [original ZT]
lang001.dll [Beach Resort Zoo (Intermediate) scenario]
lang002.dll [Christmas Tree Farm (Beginner) scenario]
lang007.dll [Reindeer]
lang008.dll [Fancy Buildings Pack]
lang009.dll [Magnet]
lang010.dll [North African Theme Pack]
lang012.dll [African Wild Dog]
lang013.dll [Dinosaur Digs: Carnivore Zoo (Intermediate) scenario]
lang015.dll [Giant Ground Sloth]
lang016.dll [Atlantean Theme Pack]
lang017.dll [Llama]
lang018.dll [Kids Dino Pack]
lang019.dll [Komodo Dragon]
lang020.dll [Marine Mania: Ultimate Zoo (Very Advanced) scenario]
lang021.dll [Marine Mania: Surf and Turf Zoo (Beginner) scenario]
lang100.dll [DD]
lang200.dll [MM]
lang300.dll [CC] or lang301.dll [Endangered Species Pack download]
Note: There was also the official lang302.dll, but its messages are included in lang300.dll and lang301.dll.
WARNING: For user created ".dll" files, it is best to get them from each ".dll" file's main site. That will be the most up-to-date version. It will also be the version that works as the designers intended, which might not be the case if a ".dll" file is downloaded from some other site than the ".dll" file's main site. Be especially careful downloading ".dll" files from sites that do not say where the ".dll" files are from. Some sites have permissions to make certain ".dll" files from other sites available, in case someone cannot get the ".dll" file from its main site. In these cases, the alternative sites will also give the ".dll" file's main site.
LANG--ZKL.dll [Zookeeper's Lounge combined ".dll"]
langZA01.dll [Zoo Admin combined ".dll" for their early downloads]
langZA03.dll [current Zoo Admin combined ".dll"]
lang-DTH.dll [Designer's Treehouse combined ".dll"]
lang--TKZTUFcombin.dll [ZTUF combined ".dll"]
LANG--ZTCDD.dll [combined ".dll" for ZT Community Download Directory and ZT Designer Guild things]
lang999-override.dll [combined ".dll" that finishes some incomplete official messages]
langdj_pluffs.dll [combined ".dll" for dj_pluffs things as they are put in there]
LANG--fern-Tek.dll [combined ".dll" for Tek/Phoenix creations configured by fern]
LANG--Jane.dll [combined ".dll" for Tek/Phoenix creations configured by Jane; the version at Tek/Phoenix is the one to use]
LANG--ZooTek.dll [combined ".dll" for Tek/Phoenix creations configured by others]
LANG--JBL.dll [combined ".dll" for some of Jeff's (JBL's) things]
langJBSPZT-Ultimate.dll [Zooty Zootycoon combined ".dll", which was JBL's old web site; hopefully these messages will be moved to LANG--JBL.dll]
langZTforU.dll [ZTforU combined ".dll"]
LANG--ZTUBelgium.dll [ZTU Belgium's combined ".dll"]
langZTC.dll [although currently does not have messages that are not in some other ".dll", this is Zoo Tycoon Center's combined ".dll"]
LANG--ZBorsato.dll [Borsato site's combined ".dll"]
langAztecThemePack_engl.dll [English version of Aztec Theme Pack that was at Zoo Tycoon ABC]
langtelephonebooth.dll [Telephone Booth]
lang_3dcinema.dll [3D-Cinema]
langJasper.dll [Inca Museum]
There are also unique ones for benches, purchase menu filters, and a fence that are considered hacks, so are not listed above. And there are also unique ones for scenarios, which are not listed above. I have not used the following, but they are lang ".dll" files that also contain unique messages and look like they will not conflict with the above:
lang-newscen.dll [scenarios]
lang_BabyTycoon.dll [purchase menu for Baby Tycoon animals]
lang_nodnol.dll [scenario and freeform map description]
lang3D-Cinema.dll [for a 3-D Cinema that is different than the 3D-Cinema in lang_3dcinema.dll]
langlumberjack.dll [original ZT Design lumberjack pack]
langPinkFence.dll [pink fence]
langSnowtigers_Objects.dll [purchase menu filter for the Snow Tigers group's objects]
langUXP-uxp-user-expansion-pack.dll [scenarios]
langztdmegamap.dll [mega map description]
All of the other lang ".dll" files I know about either have messages that are in the above ".dll" files or have conflicts with the above ".dll" files. So I do not recommend using any other lang ".dll" files other than the above. In some cases, the ".dll" file names changed. The above ".dll" file names are the most current.