feat(core,gui): gettext translations and language preference

The app, the tray and the search prompt route every user-visible string
through GNU gettext in the `pdfs` text domain, with named placeholders and
plural forms. po/ holds the template, update and build scripts, and catalogs
for de, en_GB, es, fr, it, nl, pl and pt_BR. Preferences gains a Language row
that overrides the system language from the next start (config `language`).

Packages compile the catalogs into /usr/share/locale and the desktop files
carry translated descriptions and keywords. docs/TRANSLATING.md describes
the workflow.

README.md

@@ -55,7 +55,7 @@ pdfs-app            # the desktop app
 - [Prerequisites](#prerequisites) · [Building](#building-from-source) · [Packages](#installation--packages) · [Releases](#automated-releases-cicd)
 
 **Documentation:** [Architecture](docs/ARCHITECTURE.md) · [Testing](docs/TESTING.md) ·
-[Recovery](docs/RECOVERY.md) · [Development](docs/DEVELOPMENT.md) ·
+[Recovery](docs/RECOVERY.md) · [Development](docs/DEVELOPMENT.md) · [Translating](docs/TRANSLATING.md) ·
 [Known issues](docs/BUGS.md) · [Changelog](docs/CHANGELOG.md) ·
 [Contributing](CONTRIBUTING.md) · [Security policy](SECURITY.md)
 
@@ -499,6 +499,10 @@ Bug reports, packaging fixes, and patches are welcome — see [CONTRIBUTING.md](
 the workflow and the quality gates CI enforces, and [SECURITY.md](SECURITY.md) for reporting a
 vulnerability privately.
 
+The app, tray and search prompt are available in English, German, Spanish, French, Italian, Dutch,
+Polish and Brazilian Portuguese. [docs/TRANSLATING.md](docs/TRANSLATING.md) explains how to improve
+a translation or add a language.
+
 ## License
 
 Released under the [MIT License](LICENSE).