accounts-frontend/packages/app/components/i18n/reducer.ts

import i18n from 'app/services/i18n';

import { Action } from './actions';

export interface State {
    locale: string;
}

const defaultState: State = {
    locale: i18n.detectLanguage(),
};

export default function (state: State = defaultState, { type, payload }: Action): State {
    if (type === 'i18n:setLocale') {
        return payload;
    }

    return state;
}