The servlet

HomeServlet.java

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class HomeServlet extends HttpServlet {

    public static final String LOGIN_PAGE = "/login.jsp";


    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        LanguageConfigurator languageConfigurator = new LanguageConfigurator(request, response);
        languageConfigurator.configure();
        goToLoginPage(request, response);
    }

    private void goToLoginPage(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        RequestDispatcher dispatcher = getServletContext().getRequestDispatcher(HomeServlet.LOGIN_PAGE);
        dispatcher.forward(request, response);
    }


}

LanguageConfigurator.java

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

class LanguageConfigurator {

    public static final int MAX_AGE = 3 * 30 * 24 * 3600;
    public static final String HEADER_ACCEPT_LANGUAGE = "accept-language";
    public static final String COOKIE_LANGUAGE_NAME = "langue";
    public static final String DEFAULT_LANGUAGE = "fr";


    private HttpServletRequest request;
    private HttpServletResponse response;
    private String language;


    public LanguageConfigurator(HttpServletRequest request, HttpServletResponse response) {
        this.request = request;
        this.response = response;
    }

    public void configure() throws IOException, ServletException {
        findLanguage();
        saveLanguageIntoCookie();
        saveLanguageIntoSession();
    }

    private void findLanguage() {
        String language = getLanguageFromURLParameter();
        if (isLanguageInvalid(language)) {
            language = getLanguageFromCookie();
            if (isLanguageInvalid(language)) {
                language = getLanguageFromHeader();
            }
        }
        this.language = language != null ? language : DEFAULT_LANGUAGE;
    }

    private boolean isLanguageInvalid(String language) {
        return language == null || language.isEmpty();
    }

    private String getLanguageFromHeader() {
        return request.getHeader(HEADER_ACCEPT_LANGUAGE).substring(0, 2).trim();
    }

    private void saveLanguageIntoCookie() {
        Cookie newCookieLanguage = new Cookie(COOKIE_LANGUAGE_NAME, language);
        newCookieLanguage.setMaxAge(MAX_AGE);
        response.addCookie(newCookieLanguage);
    }

    private void saveLanguageIntoSession() {
        request.getSession().setAttribute(COOKIE_LANGUAGE_NAME, language);
    }

    private String getLanguageFromURLParameter() {
        return request.getParameter(COOKIE_LANGUAGE_NAME);
    }

    private String getLanguageFromCookie() {
        return request.getCookies() != null ? findLanguageInCookies(COOKIE_LANGUAGE_NAME, request.getCookies()) : null;
    }

    private String findLanguageInCookies(String cookieLanguage, Cookie[] cookies) {
        for (int i = 0; i < cookies.length; i++) {
            String currentCookieName = cookies[i].getName();
            if (currentCookieName.equals(cookieLanguage)) {
                return cookies[i].getValue();
            }
        }
        return null;
    }
}