Currently, built-in strings can be edited/customized by editing the ConfluenceActionSupport.properties file.

However, when doing a Confluence upgrade, the potential to wipe out your customizations is possible/likely. The new upgrade may contain new or updated strings. To take advantage of these, you need to put the new file in place, and then merge any local modifications in by hand.

A more elegant solution would be to have a file outside of the Confluence installation directory (ex. the confluence.home directory). Instead of containing all strings, this file would only contain strings which the admin wants to override.