First concept

node_modules/next/dist/server/future/route-definitions/locale-route-definition.d.ts

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+import type { RouteDefinition } from './route-definition';
+import type { RouteKind } from '../route-kind';
+export interface LocaleRouteDefinition<K extends RouteKind = RouteKind> extends RouteDefinition<K> {
+    /**
+     * When defined it means that this route is locale aware. When undefined,
+     * it means no special handling has to occur to process locales.
+     */
+    i18n?: {
+        /**
+         * Describes the locale for the route. If this is undefined, then it
+         * indicates that this route can handle _any_ locale.
+         */
+        locale?: string;
+    };
+}