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.

docs/CHANGELOG.md

@@ -13,6 +13,31 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 
 Schema: **31** (unchanged).
 
+### Added
+- **Translations.** The desktop app, the tray and the search prompt are translated into German,
+  British English, Spanish, French, Italian, Dutch, Polish and Brazilian Portuguese, and follow the
+  system language. Counts use each language's plural rules, and dates use its own formats. The
+  desktop files carry translated descriptions and search keywords. The `pdfs` CLI and messages
+  from the daemon stay in English. [docs/TRANSLATING.md](TRANSLATING.md) describes how to improve a
+  translation or add a language. A few labels on the Photos page (the filter tabs, the gallery
+  subtitle and the empty timeline) are still in English.
+- **Language preference.** Preferences → General → Appearance → Language overrides the system
+  language for the app, the tray and the prompt, from their next start.
+
+### Changed
+- **Packages install translations.** The Arch, AUR, Debian and Fedora packages install the compiled
+  catalogs under `/usr/share/locale`, and the release tarball carries them in `locale/`. Building
+  the source packages now needs `gettext`.
+
+## [2.0.0] — 2026-09-23
+
+A redesign of the desktop app: a new shell with a Preferences dialog, keyboard-driven context
+menus, a docked details pane, a rewritten photo viewer with infinite scroll, album management,
+stop sharing and link expiry, a redesigned Shared with Me page, per-device restore, a tray that
+shows the sync state, and a new app icon.
+
+Schema: **31** (unchanged).
+
 ### Added
 - **Pause and resume sync.** The Sync page, the tray and `pdfs sync pause [--for 1h]` /
   `pdfs sync resume` hold back queued uploads and mirror-folder reconciles. Reads through the mount
@@ -970,6 +995,7 @@ First stable release: FUSE files-on-demand mount, sync daemon under `proton-driv
 - The outstanding FUSE defects tracked in `docs/BUGS.md`, plus a truncate defect, validated
   by a new POSIX compliance suite for the filesystem.
 
+[2.0.0]: https://github.com/narrrl/proton-drive-linux/releases/tag/v2.0.0
 [1.11.1]: https://github.com/narrrl/proton-drive-linux/releases/tag/v1.11.1
 [1.11.0]: https://github.com/narrrl/proton-drive-linux/releases/tag/v1.11.0
 [1.10.2]: https://github.com/narrrl/proton-drive-linux/releases/tag/v1.10.2

docs/TRANSLATING.md

@@ -0,0 +1,78 @@
+# Translating Proton Drive for Linux
+
+The desktop app (`pdfs-app`), the tray (`pdfs-tray`) and the search prompt (`pdfs-prompt`) are
+translated with GNU gettext in the `pdfs` text domain. The `pdfs` command-line tool and the daemon's
+own messages stay in English, because scripts parse their output.
+
+US English is the source language. The shipped catalogs are listed in [`po/LINGUAS`](../po/LINGUAS):
+German, British English, Spanish, French, Italian, Dutch, Polish and Brazilian Portuguese.
+
+## Picking a language
+
+The apps follow the system locale (`LANGUAGE`, `LC_ALL`, `LC_MESSAGES`, `LANG`). Preferences →
+General → Appearance → **Language** overrides that for the app, the tray and the prompt. The choice is
+stored as `language` in `config.json` and takes effect the next time they start.
+
+## Layout
+
+| Path | Purpose |
+| --- | --- |
+| `crates/pdfs-gui/src/i18n.rs` | Locale setup and the `gettext`, `gettext_f`, `ngettext_f`, `pgettext` and `gettext_noop` helpers |
+| `po/POTFILES.in` | Sources that `xgettext` scans |
+| `po/pdfs.pot` | The template, generated from the sources |
+| `po/<lang>.po` | One catalog per language |
+| `po/update.sh` | Regenerates the template and merges it into every catalog |
+| `po/build.sh <localedir>` | Compiles the catalogs to `<localedir>/<lang>/LC_MESSAGES/pdfs.mo` |
+
+## Updating an existing translation
+
+1. Run `po/update.sh`. It regenerates `po/pdfs.pot` and merges new and changed strings into each
+   catalog, marking changed ones `fuzzy`.
+2. Edit `po/<lang>.po` with a PO editor (Poedit, Lokalize, GNOME Translation Editor) or a text
+   editor. Translate the empty entries and review every `fuzzy` one, then remove its `fuzzy` flag.
+3. Check the catalog with `msgfmt --check --statistics -o /dev/null po/<lang>.po`.
+4. Try it from the source tree:
+
+   ```bash
+   po/build.sh target/locale
+   PDFS_LOCALEDIR=target/locale LANGUAGE=<lang> cargo run -p pdfs-gui --bin pdfs-app
+   ```
+
+## Adding a language
+
+1. Add the code (for example `sv` or `pt_PT`) to `po/LINGUAS`, keeping the list sorted.
+2. Run `po/update.sh`. It creates `po/<lang>.po` from the template with `msginit`.
+3. Fill in the `Plural-Forms` header if `msginit` did not, and translate the catalog.
+4. Add the code and the language's own name to `LANGUAGES` in `crates/pdfs-gui/src/i18n.rs`, so the
+   Preferences picker offers it.
+5. Add `GenericName[<lang>]`, `Comment[<lang>]` and `Keywords[<lang>]` lines to the two desktop files
+   in `packaging/`.
+
+`cargo test -p pdfs-gui` checks that every language in the picker has a catalog, that
+`po/POTFILES.in` lists every source file that calls gettext, and that every catalog passes
+`msgfmt --check`.
+
+## Rules for translators
+
+- Keep every `{placeholder}` from the English text. You may move it, but not rename or drop it.
+  `msgfmt --check` rejects a catalog where they differ. Plural forms keep `{n}` in every form.
+- Read the `#.` translator comments above an entry. They explain placeholders and date formats.
+- Date entries such as `%b %e` are `strftime` formats for GLib's `DateTime::format`. Translate them
+  into the order and separators your language uses, not into literal dates.
+- A `msgctxt` such as `verb` or `column` tells you which meaning of a short word is meant.
+- Keep product names as they are: Proton Drive, Proton, Google Takeout.
+- Use the same term for the same thing across the catalog, and follow the terminology of your
+  desktop (GNOME or KDE) for common actions such as Trash, Preferences and Cancel.
+
+## Rules for developers
+
+- Wrap every user-visible GUI string: `gettext("Move to Trash")`.
+- Never build a sentence from translated pieces or with `format!`. Use named placeholders:
+  `gettext_f("Moved {name} to Trash", &[("name", &name)])`.
+- Counts use `ngettext_f("{n} item", "{n} items", count, &[])`; `{n}` is filled in for you.
+- Give short, ambiguous words a context: `pgettext("verb", "Open")`.
+- Mark labels in `const` tables with `gettext_noop` and translate them where they are shown.
+- The message id must be a single string literal on one line, since `xgettext` reads literals only.
+- Add a `// Translators:` comment on the line above a call whose meaning is not obvious.
+- Do not translate log messages, CLI output, action or icon names, or values the daemon sends back.
+- Run `po/update.sh` after changing strings and commit the updated template and catalogs.