Update PlainTab I18N

Use this skill for PlainTab runtime UI copy. Keep the runtime contract unchanged:

• js/languages.js is the synchronous language bootstrap and must stay early in index.html.
• js/i18n/*.js are language packs that assign flat objects to window.I18N["locale"].
• Runtime code should call t(key) or use existing data-i18n; do not hard-code UI language branches.
• Keep all language packs key-complete. zh-CN and en need carefully maintained copy, and every supported locale should receive localized UI text for newly added runtime keys. Do not use English as the default fallback for non-English packs; only leave a product name, protocol, shortcut, or technical token in English when that exact token is the UI concept.

Workflow

1. Read .claude/rules/30-language.md and any touched runtime file.
2. Add or update flat keys in every js/i18n/*.js file. New user-facing keys must be localized for all supported locales, not copied from en.
3. Keep language packs multi-line, one key per line:

window.I18N["en"] = {
    "someKey": "Some text",
    "anotherKey": "Another text"
};

4. In runtime code, reference keys through t("someKey"). Avoid detecting language manually for UI strings.
5. For generated or mechanical updates, preserve UTF-8 text. Be careful with PowerShell pipelines that can turn non-ASCII text into ?; prefer apply_patch or an ASCII-safe temporary script that is deleted after use.
6. If adding user-facing behavior, keep first-paint rules intact; do not move js/languages.js or js/preload.js.

Validation

Run these checks before finishing:

Get-ChildItem js\i18n\*.js | ForEach-Object { node --check $_.FullName }
node --check js\languages.js
node --check js\newtab.js
@'
const fs = require('fs');
global.window = {};
global.document = { write() {} };
global.navigator = { language: 'en' };
global.localStorage = { getItem() { return ''; } };
require('./js/languages.js');
for (const file of fs.readdirSync('js/i18n').filter(f => f.endsWith('.js'))) require('./js/i18n/' + file);
const report = window.validatePlainTabI18N({ silent: true });
if (!report.ok) {
  console.log(JSON.stringify(report, null, 2));
  process.exit(1);
}
console.log('validatePlainTabI18N ok');
'@ | node -

If only specific runtime files changed, still run git diff --check on the touched files.

Also audit newly added keys for accidental English fallback in non-English packs. A value that is identical to window.I18N.en[key] should be treated as suspicious unless it is an intentional product name, protocol, shortcut, or technical token.