How to make Localization update believe that the translation of a local .po-file not up to date?

I use a local development branch of a contributed project on several sites. The strings that need translation does not exist on https://localize.drupal.org/ (yet), so I cannot use it to translate the project. Checking l10n update status produces: $ drush l10n-update-status … my_project (7.x-1.x) Up to date … Now, how do I convince Localization update […]