To find the files for the texts of the different languages, just simply follow this.
Right click on "The Forest" in your Steam library -> Manage -> Browse local files
From there just open the "lang" folder, where a bunch of .json files are located for each available language in the game.

From there you can open the file you wanna edit with any text editor.
After making your changes, just save the file, close your text editor and start the game to see your changes in the game.

To simply change the content from the .json files just simply use one of these programs, because they support the .json format.

Sublime Text[www.sublimetext.com]
Notepad++[notepad-plus-plus.org]