Hi Holly! I'm glad to see new members joining out community!
The .dll files or language files contain most messages the game uses for several items, like those that pop when you select a building or the guests thoughts... Some UCIs (user created items) however, run with messages not contained within the vanilla files, so the developers "craft" their own .dll... Nevertheless, ZT has a .dll number limit and, because of this, there are compilations of .dll that contain a merge of several message files for the game to support them in order to run several UCIs with the fewer user made .dll files... Combined files are kept in our "libraries" like ZTCDD or ZKL... Take a look on this site:
http://www.ztcdd.org/DD/index.php?board=136.0
If these sites are like a sort of libraries, our librarians are Fern & Cricket ♡♡ They may aid you with this topic more than I could!
These .dll files should be placed in the main directory of the game so the game loads them properly!
One more timen welcome to ZooTek! ??