You should edit localization messages in a different resource file from the one you configure the portlet to read messages from.
The build process combines resource files into a single resource file that the component reads messages from. The build combines the component's com/endeca/PluginResource_[locale].properties file and any file found in a shared library's directory matching com/endeca/*Resource_[locale].properties into a single com/endeca/Resource_[locale].properties file. The messages from your component's PluginResource_[locale].properties appear at the top of the final Resource_[locale].properties, so you can easily override any messages from shared libraries. However, if your component includes more than one shared library, no guarantee can be made about the order in which the resource files from shared libraries will be appended.
If you have included localized messages in your shared libraries, make sure you choose a prefix other than Plugin for the resource file com/endeca/[prefix]Resource_[locale].properties. If you do not, this file will override your component's com/endeca/PluginResource_[locale].properties file during the build, and your final com/endeca/Resource_[locale].properties will be incorrect. Endeca recommends that you choose a prefix for your library's resource file that is distinct and similar to your library's name to avoid file name conflicts with components or other shared libraries.