Please first download
Resource files for FileVista v4.1. Note that v4.0 language files are not compatible (new strings added) with v4.1. Only language and css files (no images) are included in this zip.
For instance, if you want to customize the english language file, then copy App_GlobalResources\GleamTech.FileVista\languages\en.xml from the above zip file to the corresponding location under your FileVista folder (FileVista\App_GlobalResources\GleamTech.FileVista\languages\). Then edit this xml file and change the necessary strings. Whenever you do a change under FileVista\App_GlobalResources, ASP.NET engine should restart the application so changes will be detected automatically. If changes are not detected, you can force by recycle the app-pool or touch the web.config (open and save) as Chris said.