If you plan to play the Age of Charlemagne DLC (which is usually included in the Codex exclusive release), the English files are mandatory to understand the new mission objectives.

local_en.pack (Contains all English text, menus, UI, and subtitles)

In the community, "exclusive" English files usually refer to the specific localization data needed to switch a game's text and audio from another language (like Russian or French) back to English. How to Manage English Language Files

: The actual English audio and text assets found in the game's Official Way to Change Language