The component is localized entirely in English by default, but you can override these localizations via the translation prop - a function that has a signature compatible with the t function of i18next. You only need to localize parts of the UI that you'll actually be using, there is no need to localize the entirety of the editor unless you need to. If you're using i18next, you can use browse the locales directory in GitHub for a default set of translations or use a tool like i18next Parser to extract them from the source code. If you're using another i18n library, you can use the translation prop to pass in your own translations

function LocalizedEditor() {
  return <MDXEditor translation={(key, defaultValue, interpolations) => { return i18n.t(key, defaultValue, interpolations) }} />