diff --git a/cmd/reasonix-studio-host/language_test.go b/cmd/reasonix-studio-host/language_test.go
new file mode 100644
index 0000000000..e6d0d36c5b
--- /dev/null
+++ b/cmd/reasonix-studio-host/language_test.go
@@ -0,0 +1,28 @@
+package main
+
+import (
+	"testing"
+
+	"reasonix/internal/base/i18n"
+	"reasonix/internal/contract/config"
+)
+
+func TestKernelLanguageWithShellSystemLanguage(t *testing.T) {
+	before := i18n.CurrentLanguage()
+	t.Cleanup(func() { i18n.DetectLanguage(before) })
+	for _, key := range []string{"REASONIX_LANG", "LC_ALL", "LC_MESSAGES", "LANG"} {
+		t.Setenv(key, "")
+	}
+	t.Setenv("REASONIX_SYSTEM_LANG", "zh-CN")
+	for _, tt := range []struct{ setting, env, want string }{
+		{"auto", "", "zh"},
+		{"en", "", "en"},
+		{"auto", "en", "en"},
+		{"zh-TW", "en", "zh-TW"},
+	} {
+		t.Setenv("REASONIX_LANG", tt.env)
+		if got := resolveKernelLanguage(&config.Config{Language: tt.setting}); got != tt.want {
+			t.Errorf("setting %q, env %q: language = %q, want %q", tt.setting, tt.env, got, tt.want)
+		}
+	}
+}
diff --git a/cmd/reasonix-studio-host/tray_language_effect_test.go b/cmd/reasonix-studio-host/tray_language_effect_test.go
new file mode 100644
index 0000000000..6ac0f3db95
--- /dev/null
+++ b/cmd/reasonix-studio-host/tray_language_effect_test.go
@@ -0,0 +1,100 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+	"time"
+
+	"reasonix/internal/base/i18n"
+	"reasonix/internal/base/testenv"
+	"reasonix/internal/contract/config"
+	"reasonix/internal/frontend/serve"
+)
+
+func TestAssembledTrayLanguage(t *testing.T) {
+	before := i18n.CurrentLanguage()
+	t.Cleanup(func() { i18n.DetectLanguage(before) })
+	for _, tt := range []struct{ name, system, desktop, env, locale, want string }{
+		{"system", "zh-Hans-CN", "auto", "", "", "zh"},
+		{"traditional", "zh-Hant-TW", "auto", "", "", "zh-TW"},
+		{"desktop", "zh-Hans-CN", "en", "", "", "en"},
+		{"reasonix", "zh-Hans-CN", "auto", "en-US", "", "en"},
+		{"desktop-over-env", "en-US", "zh", "en", "", "zh"},
+		{"c-locale", "zh-Hans-CN", "auto", "", "C", "en"},
+		{"posix-locale", "zh-Hans-CN", "auto", "", "POSIX.UTF-8", "en"},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Setenv("REASONIX_HOME", testenv.TempDir(t))
+			t.Chdir(testenv.TempDir(t))
+			for _, key := range []string{"REASONIX_LANG", "LC_ALL", "LC_MESSAGES", "LANG"} {
+				t.Setenv(key, "")
+			}
+			t.Setenv("REASONIX_SYSTEM_LANG", tt.system)
+			t.Setenv("REASONIX_LANG", tt.env)
+			t.Setenv("LANG", tt.locale)
+			cfg := config.Default()
+			disabled := false
+			cfg.Desktop.Telemetry, cfg.Desktop.Metrics = &disabled, &disabled
+			if err := cfg.SetDesktopLanguage(tt.desktop); err != nil {
+				t.Fatal(err)
+			}
+			if err := cfg.SaveTo(config.UserConfigPath()); err != nil {
+				t.Fatal(err)
+			}
+			ctx, cancel := context.WithCancel(t.Context())
+			defer cancel()
+			hub, err := assemble(ctx, io.Discard, io.Discard, shellIdentity{}, nil, newStartupPhases(io.Discard, time.Now))
+			if err != nil {
+				t.Fatalf("assemble: %v", err)
+			}
+			defer hub.Shutdown()
+			b, err := bind(hub.Handler())
+			if err != nil {
+				t.Fatal(err)
+			}
+			done := make(chan error, 1)
+			go func() { done <- b.serve(ctx) }()
+			defer func() {
+				cancel()
+				select {
+				case err := <-done:
+					if err != nil {
+						t.Error(err)
+					}
+				case <-time.After(30 * time.Second):
+					t.Error("host did not drain")
+				}
+			}()
+			check := func(want string) {
+				t.Helper()
+				status, body := ask(t, b, http.MethodGet, "/tray/state", nil)
+				if status != http.StatusOK {
+					t.Fatalf("tray status = %d", status)
+				}
+				var state serve.TrayState
+				if err := json.Unmarshal([]byte(body), &state); err != nil {
+					t.Fatal(err)
+				}
+				say := i18n.Catalog(want)
+				labels := serve.TrayLabels{Open: say.TrayOpen, CloseToTray: say.TrayCloseToTray, Quit: say.TrayQuit}
+				if state.Labels != labels || state.Line != say.TrayIdle {
+					t.Fatalf("tray = %+v, want labels %+v and line %q", state, labels, say.TrayIdle)
+				}
+			}
+			check(tt.want)
+			for _, lang := range []string{"en", "zh"} {
+				status, _ := ask(t, b, http.MethodPost, "/appearance", func(r *http.Request) {
+					r.Body = io.NopCloser(strings.NewReader(`{"language":"` + lang + `"}`))
+				})
+				if status != http.StatusOK {
+					t.Fatalf("appearance status = %d", status)
+				}
+				check(lang)
+			}
+		})
+	}
+}
diff --git a/desktop/electron/src/host.js b/desktop/electron/src/host.js
index 08907203db..801ec25d23 100644
--- a/desktop/electron/src/host.js
+++ b/desktop/electron/src/host.js
@@ -25,8 +25,10 @@ class HostStartError extends Error {
 // the only channel pointing this way. stdin is the lease: the kernel drains
 // when this end closes, which is what stops it outliving a parent that exited
 // without being asked to.
-function start(binary, args, { onStderr, onExit, onAct, onSlow, timeoutMs = HANDSHAKE_TIMEOUT_MS } = {}) {
-  const child = spawn(binary, args, { stdio: ["pipe", "pipe", "pipe"] });
+function start(binary, args, { onStderr, onExit, onAct, onSlow, systemLanguage, timeoutMs = HANDSHAKE_TIMEOUT_MS } = {}) {
+  const env = { ...process.env };
+  if (systemLanguage) env.REASONIX_SYSTEM_LANG = systemLanguage;
+  const child = spawn(binary, args, { stdio: ["pipe", "pipe", "pipe"], env });
   child.stderr.setEncoding("utf8");
   if (onStderr) child.stderr.on("data", onStderr);
   if (onExit) child.on("exit", onExit);
diff --git a/desktop/electron/src/main.js b/desktop/electron/src/main.js
index 9c83811237..442443b34a 100644
--- a/desktop/electron/src/main.js
+++ b/desktop/electron/src/main.js
@@ -94,6 +94,7 @@ async function launchKernel(args) {
     const began = Date.now();
     logs.shell.line(`host: starting ${hostBinary} (attempt ${attempt})`);
     kernel = start(hostBinary, args, {
+      systemLanguage: app.getPreferredSystemLanguages()[0] ?? app.getLocale(),
       timeoutMs: handshakeTimeout(),
       onSlow: () => {
         logs.shell.line("host: no handshake yet; showing the starting window");
diff --git a/desktop/electron/test/unit.mjs b/desktop/electron/test/unit.mjs
index 098743961b..d88d5b1fa6 100644
--- a/desktop/electron/test/unit.mjs
+++ b/desktop/electron/test/unit.mjs
@@ -6,7 +6,7 @@ import os from "node:os";
 import fs from "node:fs";
 
 const require = createRequire(import.meta.url);
-const { parse, readActs } = require("../src/host.js");
+const { parse, readActs, start } = require("../src/host.js");
 const { contextTemplate, editMenuTemplate, applicationMenuTemplate, menuInstaller } = require("../src/editmenu.js");
 const { uiLanguage } = require("../src/uilang.js");
 const { externalTarget } = require("../src/links.js");
@@ -17,6 +17,23 @@ const { pick, loadPrefs, savePrefs, registerPrefs } = require("../src/prefs.js")
 const TOKEN = "a".repeat(64);
 const line = (over) => JSON.stringify({ version: 1, origin: "http://127.0.0.1:8080", token: TOKEN, ...over });
 
+test("the spawned host receives the system language and inherits explicit locale overrides", async () => {
+  const script = `console.log(${JSON.stringify(line())});
+    console.log(JSON.stringify({act: JSON.stringify({system: process.env.REASONIX_SYSTEM_LANG,
+      explicit: process.env.REASONIX_LANG})}));`;
+  const before = process.env.REASONIX_SYSTEM_LANG;
+  const explicit = process.env.REASONIX_LANG;
+  const state = new Promise((resolve) => {
+    const host = start(process.execPath, ["-e", script], {
+      systemLanguage: "zh-Hant-TW",
+      onAct: (act) => resolve(JSON.parse(act)),
+    });
+    host.ready.catch(resolve);
+  });
+  assert.deepEqual(await state, { system: "zh-Hant-TW", ...(explicit === undefined ? {} : { explicit }) });
+  assert.equal(process.env.REASONIX_SYSTEM_LANG, before);
+});
+
 test("the handshake is accepted only when every field accounts for itself", () => {
   assert.deepEqual(parse(line()), { origin: "http://127.0.0.1:8080", token: TOKEN });
 
diff --git a/internal/base/i18n/detect.go b/internal/base/i18n/detect.go
index 81defa7382..b5a4c8bdfc 100644
--- a/internal/base/i18n/detect.go
+++ b/internal/base/i18n/detect.go
@@ -5,11 +5,9 @@ import (
 	"strings"
 )
 
-// DetectLanguage selects a catalogue from override (e.g. cfg.Language) or the
-// environment and installs it as M. Returns the resolved tag ("en", "zh") so
-// callers can log or expose it.
-//
-// Priority: override > REASONIX_LANG > LC_ALL > LC_MESSAGES > LANG > "en".
+// DetectLanguage installs the catalogue for an explicit setting or locale.
+// Explicit settings and locale variables take priority over the shell's system
+// language and the native OS probe; unresolved locales fall back to English.
 func DetectLanguage(override string) string {
 	for _, c := range append([]string{override}, envCandidates()...) {
 		if tag := normalize(c); tag != "" {
@@ -24,7 +22,7 @@ func DetectLanguage(override string) string {
 var osLanguage = detectOSLanguage
 
 func envCandidates() []string {
-	keys := []string{"REASONIX_LANG", "LC_ALL", "LC_MESSAGES", "LANG"}
+	keys := []string{"REASONIX_LANG", "LC_ALL", "LC_MESSAGES", "LANG", "REASONIX_SYSTEM_LANG"}
 	out := make([]string, 0, len(keys)+1)
 	for _, k := range keys {
 		out = append(out, os.Getenv(k))
@@ -58,6 +56,9 @@ func normalize(s string) string {
 	if s == "" {
 		return ""
 	}
+	if s == "c" || s == "posix" || strings.HasPrefix(s, "c.") || strings.HasPrefix(s, "posix.") {
+		return "en"
+	}
 	if strings.HasPrefix(s, "zh-tw") || strings.HasPrefix(s, "zh-hant") || strings.Contains(s, "chinese traditional") || strings.Contains(s, "繁體") {
 		return "zh-TW"
 	}
diff --git a/internal/base/i18n/locale_test.go b/internal/base/i18n/locale_test.go
new file mode 100644
index 0000000000..41df5eeaa5
--- /dev/null
+++ b/internal/base/i18n/locale_test.go
@@ -0,0 +1,49 @@
+package i18n
+
+import "testing"
+
+func TestNormalizePOSIXLocales(t *testing.T) {
+	for _, tt := range []struct{ locale, want string }{
+		{"C", "en"}, {"POSIX", "en"}, {"C.UTF-8", "en"},
+		{"C.ISO8859-1", "en"}, {"POSIX.UTF-8", "en"}, {"POSIX.ASCII", "en"},
+		{"  c.utf-8  ", "en"}, {"", ""}, {"   ", ""},
+		{"zh-Hans-CN", "zh"}, {"zh_TW.UTF-8", "zh-TW"}, {"en-US", "en"},
+		{"ca-ES", ""}, {"posix-extra", ""},
+	} {
+		t.Run(tt.locale, func(t *testing.T) {
+			if got := normalize(tt.locale); got != tt.want {
+				t.Fatalf("normalize(%q) = %q, want %q", tt.locale, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestPOSIXLocaleWinsOverSystemLanguage(t *testing.T) {
+	before, probe := CurrentLanguage(), osLanguage
+	osLanguage = func() string { return "zh-TW" }
+	t.Cleanup(func() { setLanguage(before); osLanguage = probe })
+	for _, key := range []string{"LC_ALL", "LC_MESSAGES", "LANG"} {
+		for _, locale := range []string{"C", "POSIX", "C.UTF-8", "C.ASCII", "POSIX.UTF-8"} {
+			t.Run(key+"/"+locale, func(t *testing.T) {
+				for _, candidate := range []string{"REASONIX_LANG", "LC_ALL", "LC_MESSAGES", "LANG"} {
+					t.Setenv(candidate, "")
+				}
+				t.Setenv("REASONIX_SYSTEM_LANG", "zh-Hans-CN")
+				t.Setenv(key, locale)
+				if got := DetectLanguage("auto"); got != "en" {
+					t.Fatalf("language = %q, want en", got)
+				}
+				if got := CatalogFor("auto").TrayOpen; got != English.TrayOpen {
+					t.Fatalf("tray open = %q, want %q", got, English.TrayOpen)
+				}
+				t.Setenv("REASONIX_LANG", "zh-TW")
+				if got := DetectLanguage("auto"); got != "zh-TW" {
+					t.Fatalf("environment override = %q, want zh-TW", got)
+				}
+				if got := DetectLanguage("zh"); got != "zh" {
+					t.Fatalf("explicit override = %q, want zh", got)
+				}
+			})
+		}
+	}
+}
diff --git a/internal/base/i18n/system_language_test.go b/internal/base/i18n/system_language_test.go
new file mode 100644
index 0000000000..9f25ad041d
--- /dev/null
+++ b/internal/base/i18n/system_language_test.go
@@ -0,0 +1,42 @@
+package i18n
+
+import "testing"
+
+func TestShellSystemLanguagePriority(t *testing.T) {
+	probe := osLanguage
+	osLanguage = func() string { return "en-US" }
+	t.Cleanup(func() { osLanguage = probe })
+	before := CurrentLanguage()
+	t.Cleanup(func() { setLanguage(before) })
+	for _, tt := range []struct {
+		name, override, key, value, system, want string
+	}{
+		{"auto", "auto", "", "", "zh-CN", "zh"},
+		{"traditional", "auto", "", "", "zh-Hant-TW", "zh-TW"},
+		{"setting", "en", "", "", "zh-CN", "en"},
+		{"reasonix", "auto", "REASONIX_LANG", "en", "zh-CN", "en"},
+		{"all", "auto", "LC_ALL", "en_US.UTF-8", "zh-CN", "en"},
+		{"messages", "auto", "LC_MESSAGES", "en_US.UTF-8", "zh-CN", "en"},
+		{"lang", "auto", "LANG", "en_US.UTF-8", "zh-CN", "en"},
+		{"unsupported", "auto", "", "", "fr-FR", "en"},
+		{"absent", "auto", "", "", "", "en"},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			for _, key := range []string{"REASONIX_LANG", "LC_ALL", "LC_MESSAGES", "LANG"} {
+				t.Setenv(key, "")
+			}
+			t.Setenv("REASONIX_SYSTEM_LANG", tt.system)
+			if tt.key != "" {
+				t.Setenv(tt.key, tt.value)
+			}
+			if got := DetectLanguage(tt.override); got != tt.want {
+				t.Fatalf("language = %q, want %q", got, tt.want)
+			}
+			say := CatalogFor(tt.override)
+			want := Catalog(tt.want)
+			if say.TrayOpen != want.TrayOpen || say.TrayCloseToTray != want.TrayCloseToTray || say.TrayQuit != want.TrayQuit || say.TrayIdle != want.TrayIdle {
+				t.Fatal("tray catalogue does not follow the resolved language")
+			}
+		})
+	}
+}