Set this to path of a folder with structure similar to Root Folder.
On each translation file open, a corresponding file in Alternate translations directory will be looked up and,
if it is found, it will be used for Alternate Translations view.
You will likely want to use translations of another target language (i.e. another subproject), which is close to yours.
Also, you can use other subproject's translations either immediately, or have them pretranslated to your language by machine.
See documentation for more details.