To install additional language dictionaries for Fade In Professional Screenwriting Software, download the appropriate file(s) from one of the listed sites. You may need to extract/unzip the downloaded package: what you want are both .dic and .aff files for any given language.
(Normally, depending on your operating system, you can extract a zip package by double-clicking on it, or by right-clicking and choosing the option to extract.)
From an open document in the application, select Document > Check Spelling, and from the language choice drop-down, selected Install/remove.
Click the Install button, and select the dictionary (.dic) file that you want to install. (Repeat for multiple languages, if desired.)
Note that some dictionaries are distributed as .oxt Open Office extensions or .xpi Mozilla add-ons. In order to open these, just rename to .zip, and open as you would a zip archive to find the .dic and .aff dictionary files.