You can download a locale in XML format for offline editing. It includes entries for all files in root, including addons, and in all themes. Any local edits will be included in the download.
You can upload the edited XML file via the 'Locale XML Upload' feature or via the command line script `/scripts/locale_import --import=path/to/your/file.xlf` (pass it --help for more information).
If a locale seems to be missing you may need to rebuild the locale databases in order to bring it into the system.