Play Framework 2 Language Code in URL Concept?

You can implement custom request handler and resolve language on every request. This is the same idea like your "in each controller call a method for setting the language" but you need to write the code only in one place - legacy GlobalSettings.onRequest or new HttpRequestHandler.createAction

There is a very good description about realisation i18n in play based on the url part, the only one thing - it's for 2.0.4, so I suppose you would use HttpRequestHandler.createAction but GlobalSettings.onRequest.

Guide: http://www.flowstopper.org/2013/01/i18n-play-framework-java-app-website.html

Migration guide: https://www.playframework.com/documentation/2.4.x/GlobalSettings

Custom Request Handlers: https://www.playframework.com/documentation/2.4.x/JavaHttpRequestHandlers

Live examples from my project (Play 2.4.3, Java)

application.conf

play.i18n.langs = [ "en", "de", "fr", "ua" ]
play.http.requestHandler = "plugins.RequestHandler"

routes

# Home page
GET     /$lang<[a-z]{2}>/home       controllers.Application.home(lang:String)

plugins/RequestHandler.java

package plugins;

import play.http.DefaultHttpRequestHandler;
import play.libs.F;
import play.mvc.Action;
import play.mvc.Http;
import play.mvc.Result;

import java.nio.file.Path;
import java.nio.file.Paths;
import java.lang.reflect.Method;

public class RequestHandler extends DefaultHttpRequestHandler {

    @Override
    public Action createAction(Http.Request request, Method actionMethod) {
        return new Action.Simple() {
            @Override
            public F.Promise<Result> call(Http.Context ctx) throws Throwable {
                Path path = Paths.get(ctx.request().path());
                String lang = path.getName(0).toString();
                // we detect language only by URL path, cookies does not used 
                ctx.setTransientLang(lang);
                return delegate.call(ctx);
            }
        };
    }
}

controllers/Application.java

package controllers;

import play.*;
import play.mvc.*;
import play.i18n.Lang;

import views.html.*;

public class Application extends Controller {

    public Result home(String lang){
       return ok(ctx().lang().code());
    }

}

This App would give results

http://localhost:9000/de/home -> "de"

http://localhost:9000/en/home -> "en"

http://localhost:9000/dk/home -> "exception: Language not supported in this application: Lang(dk,) not in Lang.availables()"

Take in to attention: Lang.defaultLang().language() will not return current request language. You need to call ctx().lang() for returning current request language.