The download that is in the "Lang MSBF" post at the ZA ZT1 download site contains 3 files: a ".dll", a ".ztd", and a ".txt" file that explains how to use the ".dll". There were a number of official downloads. These official downloads were also included in CC. In both cases, there were 15 ".dll" files that were made available to make those downloads work. The lang050MS.dll is just a ".dll" file that combines these 15 separate ".dll" files. Since there are enough other combined ".dll" files from various sites that I do not have to worry about reaching ZT's ".dll" limit, I do not use lang050MS.dll. But it is nice to know it is there if ever needed.
I do not fully understand the yeti01.ztd. One of the changes was to change the yeti's habitat from Tundra to Highland. According to the description in the download post, it looks like it was also intended that the yeti would no longer appear in the animal purchase menu of scenarios. I do not know if it actually does this, since I do not play scenarios. And the configuration looks like there is an inconsistency. In any case, this change causes it to want to get messages from ".dll" files instead of from the animal configuration. So if you use it without lang050MS.dll, you will see "Adopt a Yeti. Long Tooltip placeholder text." as its purchase menu tooltip. But if you use lang050MS.dll, you will see "Adopt an Yeti. Yetis are elusive, solitary creatures. They prefer to sleep in snowy rock caves." instead.