Add Simplified Chinese as a system language

The installer asks for the language before the keyboard, and
Setup > Language (omarchy language en|zh) switches it later. Under
zh_CN the menu reads its labels from omarchy-menu.zh-cn.jsonc, the
shell from I18nZh.js through I18n.tr, and notifications are worded
by their sender through omarchy-text, which reads the same table.
User text is never translated.

test/shell.d/typing-setup-checks.py

@@ -74,7 +74,8 @@ class TypingSetupTest(unittest.TestCase):
 
   def test_an_uninstalled_engine_names_its_package_without_restarting(self):
     absent = subprocess.CompletedProcess([], 1)
-    with patch.object(typing.setup, "live_group", return_value=("Default", "us", [["keyboard-us", ""]])), patch.object(typing.setup, "available_methods", return_value=[]), patch.object(typing.subprocess, "run", return_value=absent), patch.object(typing.setup, "run") as run, patch.object(typing, "set_inputs") as save:
+    english = lambda text, *args: text.replace("%1", str(args[0]) if args else "").replace("%2", str(args[1]) if len(args) > 1 else "")
+    with patch.object(typing.setup, "live_group", return_value=("Default", "us", [["keyboard-us", ""]])), patch.object(typing.setup, "available_methods", return_value=[]), patch.object(typing.subprocess, "run", return_value=absent), patch.object(typing, "text", side_effect=english), patch.object(typing.setup, "run") as run, patch.object(typing, "set_inputs") as save:
       with self.assertRaisesRegex(RuntimeError, "Install fcitx5-mozc to use Japanese"):
         typing.save_inputs(["mozc"])
     run.assert_not_called()