next.js/examples/with-lingui/pages/_app.js

import { useEffect } from "react";

import { I18nProvider } from "@lingui/react";
import { i18n } from "@lingui/core";
import { useRouter } from "next/router";
import { en, sv } from "make-plural/plurals";
import LangSwitcher from "../components/LangSwitcher";

i18n.loadLocaleData("en", { plurals: en });
i18n.loadLocaleData("sv", { plurals: sv });

export default function Page({ Component, pageProps }) {
  const { locale } = useRouter();

  useEffect(() => {
    async function load(locale) {
      const { messages } = await import(`../locale/${locale}/messages.po`);

      i18n.load(locale, messages);
      i18n.activate(locale);
    }

    load(locale);
  }, [locale]);

  return (
    <I18nProvider i18n={i18n}>
      <div>
        <LangSwitcher />
        <Component {...pageProps} />
      </div>
    </I18nProvider>
  );
}