For custom TEI Publisher-based apps ideal i18n customization and fallback scenario would involve TEI Publisher backend and local project backend: * [x] TEI Publisher backend (all tei-publisher labels and their community-maintained translations) * [x] Local project backend (for customized labels) * [x] `app` custom app namespace for project-specific labels * [ ] `common` for overwriting tei-publisher labels NB TEI Publisher v6.0.0 already implements double-backend scenario and custom project labels. Only overwriting tei-publisher labels is missing