wp-calypso/client/state/selectors/get-current-locale-slug.js

import { get } from 'lodash';

import 'calypso/state/ui/init';

/**
 * Gets the current ui locale slug
 * @param {Object} state - global redux state
 * @returns {string} current state value
 */
export default function getCurrentLocaleSlug( state ) {
	return get( state, 'ui.language.localeSlug', null );
}