The String Translation module (i18n_string.module, part of the i18n project) allows translation of user-defined strings. This is an API module (that is, it doesn't do anything by itself) that must be enabled only when required by other modules in the Internationalization package.
Module developers: See how this can be used in other modules without introducing new dependencies: Using the i18n API.
The string translation UI is integrated into the localization system. All strings will be translatable through the Administer > Site building > Translate interface
You must decide on the site default language before using this feature. Changing it later will garble all your user defined strings translations. Read below!
About Drupal Strings
The Drupal localization system allows translation of hardcoded strings, using locale module and .po files. However, user-defined strings (menu items, taxonomy terms, field names, etc.) are not translatable using Drupal 6 core. This is why the Internationalization package includes its own string translation layer, which keeps track and allows translation of most user-defined strings.
String Translation