The language files are in the languages subdirectory of the WebCenter installation on the Web Server, such as \Artios\WebCenter\WebServer\tomcat\webapps\<site name>\languages.
They are named wcstrings_xx.xml where xx is a two-letter code for the language, for example en for English, fr for French, and de for German.
For example, add the string <NLanguage_da>Danish</NLanguage_da> to install a Danish strings file, substituting the word for Danish in the appropriate language for the string file.