test(ci): add exporter heading levels and a11y typography smoke; update MAIN_PLAN

lib/i18n/index.ts

@@ -0,0 +1,23 @@
+type Messages = Record<string, Record<string, string>>
+
+export function createI18n({ defaultLocale, messages }: { defaultLocale: string; messages: Messages }) {
+  function format(template: string, params?: Record<string, string | number>) {
+    if (!params) return template
+    return template.replace(/\{(\w+)\}/g, (_, k) => (params[k] !== undefined ? String(params[k]) : `{${k}}`))
+  }
+
+  function getMessage(locale: string, key: string): string | undefined {
+    const m = messages[locale] || {}
+    if (m[key] !== undefined) return m[key]
+    const base = messages[defaultLocale] || {}
+    return base[key]
+  }
+
+  function t(locale: string, key: string, params?: Record<string, string | number>): string {
+    const msg = getMessage(locale, key)
+    if (typeof msg === 'string') return format(msg, params)
+    return key
+  }
+
+  return { t }
+}