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+# Supporting Multiple Languages in the Web UI
+
+The firmware uses a client-side i18n approach
+all translations are stored in a single JavaScript dictionary embedded in the firmware.
+No server-side changes are needed.
+
+## Architecture
+
+All translation logic lives in `html/i18n.js`. The file contains:
+
+- A `LANG` object with one sub-object per language (`en`, `ja`, ...)
+- Language auto-detection (browser language → `localStorage` override)
+- `t(key)` — look up a translated string
+- `setLang(lang)` — switch language and update the page
+- `applyTranslation(el)` — apply translation to one DOM element
+
+Translation keys are **flat strings** (no nesting). The English keys in `LANG.en` also serve as the fallback when a key is missing in another language.
+
+## How to Add a New Language
+
+### 1. Add a dictionary entry in `html/i18n.js`
+
+Append a new sub-object to the `LANG` object. Every key from `LANG.en` must be present:
+
+```js
+var LANG = {
+  en: {
+    nav_overview: 'Overview',
+    nav_port_config: 'Port Configuration',
+    // ... all keys for English
+  },
+  ja: {
+    nav_overview: '概要',
+    nav_port_config: 'ポート設定',
+    // ... all keys for Japanese
+  },
+  LANGCODE: {            // ← add your language here
+    nav_overview: '...',
+    nav_port_config: '...',
+    // ... translate every key
+  },
+};
+```
+
+### 2. Add the language to the navigation sidebar
+
+In `html/navigation.js`, add an `<option>` to the language selector:
+
+```js
++ "<option value='en'>English</option><option value='ja'>日本語</option>"
+```
+
+Replace with:
+
+```js
++ "<option value='en'>English</option><option value='ja'>日本語</option><option value='LANGCODE'>Native Name</option>"
+```
+
+### 3. Verify auto-detection
+
+The language detection code in `i18n.js` reads `navigator.language` and normalises it to the first two characters:
+
+```js
+var browser = (navigator.language || navigator.userLanguage || 'en').substring(0, 2);
+return LANG[browser] ? browser : 'en';
+```
+
+If the two-letter code matches a key in `LANG`, it will be auto-selected. No changes needed here.
+
+## Two Translation Mechanisms
+
+### (A) `data-i18n` attribute (declarative — for HTML)
+
+Add `data-i18n="key_name"` to any HTML element. The English text goes in the element content as a fallback:
+
+```html
+<h1 data-i18n="port_heading">Port Configuration</h1>
+<input type="button" data-i18n="port_apply" value="Apply">
+<option data-i18n="port_auto">Auto</option>
+<title data-i18n="port_title">Port Configuration</title>
+```
+
+On page load, `applyTranslation()` sets:
+
+- `el.value` for `<input type="submit|button">`
+- `el.textContent` for `<option>`, `<title>`
+- `el.innerHTML` for everything else
+
+### (B) `t('key')` call (imperative — for JavaScript strings)
+
+When generating HTML or text in JavaScript, wrap translatable strings with `t()`:
+
+```js
+td.appendChild(document.createTextNode(t('common_port') + i));
+td.innerHTML = t('port_auto');
+iHTML += "<tr><td>" + t('port_vendor') + "</td></tr>";
+```
+
+### Special Case: Link Speed Display
+
+The `linkS` array in `html/main.js` maps numeric link states to display strings. The first two entries (`speed_disabled`, `speed_down`) use `t()` for translation; the remaining entries are static literals (they are the same in all languages):
+
+```js
+const linkS = [
+  function(){return t('speed_disabled')},
+  function(){return t('speed_down')},
+  "10M", "100M", "1000M", "500M", "10G", "2.5G", "5G"
+];
+function linkText(idx) { var v = linkS[idx]; return typeof v === 'function' ? v() : v; }
+```
+
+Always use `linkText(idx)` (not `linkS[idx]`) to read these values.
+
+## Size Considerations
+
+- `html/i18n.js` is embedded in the firmware filesystem (~14 KB for two languages)
+- Each new language adds roughly the same number of bytes as the English dictionary (~3–4 KB)
+- The firmware binary is padded to 512 KiB, so a few extra KB do not change the flash footprint
+- Values that are identical in all languages should be inlined as literals rather than added to the dictionary (e.g., `"10M"`, `"2.5G"`, `"MAC"`, `"VLAN"`, `"CPU"`)
+
+## Build
+
+No special flags are needed. The `html/` directory is embedded by `fileadder` during the build.
+
+## Script Load Order
+
+`i18n.js` must be loaded after `main.js` (which defines `t()`'s dependencies like `LANG`) but before any page-specific JS that calls `t()`:
+
+```html
+<script src="/main.js"></script>
+<script src="/i18n.js"></script>
+<script src="/eee.js"></script>    <!-- uses t() -->
+```
+
+The `navigation.js` script is loaded last (bottom of `<body>`).