This updates the fallback locale for `locale: false` to be the `defaultLocale` instead of the currently active `locale` as it allows passing through URLs more seamlessly as a URL without the locale prefixed can be treated as the `defaultLocale` instead of having to worry if the locale matches the currently active locale. This also ensures `locale={false}` is tested in the i18n-support-catchall suite

Closes: https://github.com/vercel/next.js/issues/19048