create:
_errror.tsx in pages/

import { FC } from "react";
import serveProps from "@lib/serveProps";

interface ErrorProps {
  statusCode: number;
  main: any;
}

const Error: FC<ErrorProps> = ({ statusCode }) => {
  return (
    <p>
      {statusCode
        ? `An error ${statusCode} occurred on server`
        : "An error occurred on client"}
    </p>
  );
};

export const getServerSideProps = async (context: any) => {
  const statusCode = context.res
    ? context.res.statusCode
    : context.err
    ? context.err.statusCode
    : 404;

  const main = await serveProps(context);
  return {
    props: {
      statusCode,
      main,
      app: {
        theme: "light",
      },
    },
  };
};

export default Error;

then you can use getServerSideProps but it will not be static generated.

no you can get language from cookie or JWT token or whatever and pass it to whatever you need.