accounts-frontend/src/components/i18n/actions.js

// @flow
import i18n from 'services/i18n';

export const SET_LOCALE = 'i18n:setLocale';
export function setLocale(desiredLocale: string) {
    return async (dispatch: (action: Object) => any): Promise<string> => {
        const locale = i18n.detectLanguage(desiredLocale);

        dispatch(_setLocale(locale));

        return locale;
    };
}

function _setLocale(locale: string) {
    return {
        type: SET_LOCALE,
        payload: {
            locale,
        },
    };
}