Editing text
The Alien Isolation Mod Tools provide a text editor to modify any text string in the game for every supported language. As well as subtitles, the text strings cover the contents of the Sevastolink terminals, and UI strings.
To get to the string editor, navigate to the "Experimental Tools" window and select "Edit In-Game Text".
All available strings are shown in the left panel of the text editor window, with language options and string contents displayed on the right.
Strings are separated into a variety of categories in the left panel, with Sevastolink content and UI strings being found mainly within the "UI" category. A range of mission-specific categories are available for subtitles - the mission number of which relates to the game's internal mission ID system, and not the mission names in the shipped game. Check out the Mission IDs page to understand this system.
Editing a string is simple - first, select a language from the dropdown at the top right (by default, English is chosen). You can then select a string from one of the available categories. Upon clicking the string its content will be displayed in the text box on the right side of the UI.
Edit the text as intended, and press "Save".
This edit will apply to the selected language, and to apply the edit to all languages, you must work your way through them all individually.