[256051]

The window currently hardcodes English. This adds an English / 简体中文 language chooser that applies immediately without restarting a scan or losing navigation, marks, or the selected source. Refs #111.

• Open Language / 语言 from the top bar, startup/source chooser, or macOS menu; F2, Tab/Shift-Tab, Enter, Escape, and pointer selection are supported.
• Save the language atomically in a separate per-user language file beside the existing power preference. English remains the default and fallback. Save failures keep the session usable and show a warning.
• Localize window controls, progress, category/reclaim labels, relative times, Git summaries, review/deletion confirmation, help, and macOS menus using a small built-in catalog (336 messages), with no new dependencies. The catalog and docs/translations.md provide an entry point for additional languages.

Scanning, deletion guards, SSH protocol, paths, CLI flags, and dependency versions are unchanged. CLI help, exported prompts/scripts, and raw filesystem/SSH diagnostics retain their original wording.

Validation

On Ubuntu 24.04 under WSL, Rust 1.97.1:

• cargo xtask lint passed.
• setpriv --bounding-set=-dac_override,-dac_read_search cargo xtask test passed: 271 tests passed, 9 existing tests ignored. This WSL session runs as root; dropping the two permission-bypass capabilities is necessary for the existing unreadable-directory test to exercise real permission denial. That test fails under unrestricted root on the unchanged scanner.
• cargo build --locked -p disktree-app passed.
• New window tests cover keyboard and pointer language selection, cancellation, persistence/save failure, returning to the source chooser, preserved scan/marks, Chinese help/review/deletion dialogs, and controls staying inside a 900×600 window. Catalog tests check ordering, fallback, and placeholder preservation; formatting tests ensure inserted paths containing braces remain literal.

Windows and macOS have not been tested on physical machines; their existing CI jobs remain the platform checks.