This article explains exactly what this file is, why you need it, where to find it, and how to install it safely.
Editing game archives always carries risk. Before replacing or altering eng0.rda : Anno 1404- Venice ENG Language Pack -eng0.rda-
For Steam users: C:\Program Files (x86)\Steam\steamapps\common\Anno 1404 For GOG users: C:\GOG Games\Anno 1404 Gold Edition This article explains exactly what this file is,