M7: localization layer (#128)

UI strings come from per-language TOML catalogs compiled in (English text as the key and fallback, named placeholders, one/other plurals, context suffixes); command labels are translated at display time while ids, CLI, MCP and control output stay English; accessibility names follow. Preferences > Language (System / English / Español, live; the system language from the OS or the browser), pref.language for scripts. A key check test guards the catalogs. The Spanish catalog is a machine draft awaiting native review; docs/translating.md explains adding a language.

crates/engine/src/cmd/view.rs

@@ -241,6 +241,18 @@ pub fn specs() -> Vec<CommandSpec> {
             s.prefs.user_name = if name.is_empty() { super::comment::DEFAULT_AUTHOR.into() } else { name };
             Ok(json!({"name": s.prefs.user_name}))
         }),
+        spec!("pref.language", "Interface language", "{language: system|en|es|… (a language code; no catalog: English)}", false, |s, p| {
+            let lang = req_string("pref.language", p, "language")?;
+            let lang = lang.trim();
+            // A BCP 47-style tag ("es", "pt-BR", "zh_Hans") or "system".
+            let tag = lang.len() <= 35
+                && lang.split(['-', '_']).all(|part| !part.is_empty() && part.len() <= 8 && part.chars().all(|c| c.is_ascii_alphanumeric()));
+            if !tag {
+                return Err(bad("pref.language", format!("`{lang}` is not a language code")));
+            }
+            s.prefs.language = lang.to_string();
+            Ok(json!({"language": s.prefs.language}))
+        }),
         spec!("app.preferences", "Preferences", "{}", false, |s, _| ui(s, "dialog.preferences")),
         spec!("app.actions", "Actions", "{}", false, |s, _| ui(s, "palette.actions")),
         spec!("help.shortcuts", "Keyboard shortcuts", "{}", false, |s, _| ui(s, "panel.shortcuts")),

crates/engine/src/lib.rs

@@ -481,6 +481,9 @@ pub struct Prefs {
     pub number_keys_opacity: bool,
     pub show_dimensions: bool,
     pub theme: String,
+    /// The interface language: a language code ("en", "es") or "system" (the OS or browser
+    /// language). Languages without a catalog show English.
+    pub language: String,
     /// The name comments are signed with (and whose comments count as yours).
     pub user_name: String,
     /// Reduce motion: the interface's own animations are off (the system's setting turns them off
@@ -512,6 +515,7 @@ impl Default for Prefs {
             number_keys_opacity: true,
             show_dimensions: true,
             theme: "light".into(),
+            language: "system".into(),
             user_name: cmd::comment::DEFAULT_AUTHOR.into(),
             reduce_motion: false,
             invite_links: None,