setting the subdomain for english language

Check the responses and examples in this SO topic.

Is this logic correct? I changed a function

add_filter('locale', 'lang_by_subdomain2');
function lang_by_subdomain2( $lang ) {
   if (0 === strpos( $_SERVER['HTTP_HOST'], 'de.')) {
      return 'de_DE';

if (0 === strpos( $_SERVER['HTTP_HOST'], 'en.')){

return 'en_EN';

 }else {
      return $lang;
   }

   } else {
      return $lang;
   }
}

No, you don’t want to nest the conditionals. As it is the English part will never execute because it’s within the German conditional which immediately returns ‘de_DE’.

Because a return statement terminates any further execution within the function, you can simply do something like this:

function lang_by_subdomain2( $lang ) {
   if (0 === strpos( $_SERVER['HTTP_HOST'], 'de.')) return 'de_DE';
   if (0 === strpos( $_SERVER['HTTP_HOST'], 'en.')) return 'en_EN';
   return $lang;
}

This way it’s much easier to add additional languages as desired without needing to work out where the curly braces belong.

I tryed it but it’s not working. Or I should add something?

IDK, works for me. Did you retain the add filter line?
add_filter('locale', 'lang_by_subdomain2');

FYI, when I give examples, it’s only for the applicable portion, it is very rarely complete code. I assume (probably wrongly at times) others know enough to understand how to work my example into their overall code. Apologies for any confusion I may have caused.

Ok, it works. I just have to modify specific funcions such as

add_filter('the_title', function( $title, $post_id ) {
   $locale = get_locale();
   if ('de_DE' == $locale ) {
     $title_de = get_post_meta( $post_id, 'title_de', true );
     if ( ! empty( $title_de )) {
        return $title_de;
     } else {
        return $title;
     }
   } else {
     return $title;
   }
}, 10, 2 );

into

add_filter('the_title', function( $title, $post_id ) {
  $lang = substr( get_locale(), 0, 2 );
if ('it' != $lang )
   { $title_$lang = get_post_meta( $post_id, 'title_$lang', true );
     if ( ! empty( $title_$lang )) {
        return $title_$lang;
     } else {
        return $title;
     }
   } else {
     return $title;
   }
}, 10, 2 );

the second sounds good?

Yes! That’s the right approach.

there must be something wrong in my function because I broke the site ??

I was just looking at your code conceptually before — that part’s good. Specific syntax not so much ??
'title_$lang' must instead use double quotes instead of single: "title_$lang"

You have variable $title_$lang in a few places. You cannot have the second $ in that. $ is only allowed once at the very start of variable names. Note that "title_$lang" is not a variable, it’s a string that has a variable component: $lang. Despite similarity the two are not the same.

I tryed to correct but I have still a permanent error:

add_filter('the_title', function( $title, $post_id ) {
  $lang = substr( get_locale(), 0, 2 );
if ('it' != $lang )
   { title_$lang = get_post_meta( $post_id,"title_$lang", true );
     if ( ! empty( title_$lang )) {
        return title_$lang;
     } else {
        return $title;
     }
   } else {
     return $title;
   }
}, 10, 2 );

The quoted string "title_$lang" is fine. The variable name title_$lang is not, should be $title_lang for all occurrences.

if you mean in this line { title_$lang = get_post_meta( $post_id,"title_$lang", true ); the change into { $title_lang = get_post_meta( $post_id,"title_$lang", true ); is not working, or I have to do the same also inside function (){ ?

Change all occurrences of title_$lang anywhere to $title_lang except for the quoted one that’s passed to get_post_meta().

I’m trying to do the same for the menu items, so I will filter items in any language, but I have a permanent error, this is due to a “1 more closing parenthesis” but I cant see where is the error(s)

add_filter('nav_menu_item_title', 'sac_de_menu_title', 20, 2 );
function sac_de_menu_title( $title, $item_post ) {

$lang = substr( get_locale(), 0, 2 );

   if ('it' != $lang ) {
    $title_lang = get_post_meta( $item_post->ID, "title_$lang", true );
      if ( ! empty( $title_lang )) {
        return $title_lang;
     } else {
        return $title;
     }
   } else {
     return $title;
   }
}, 10, 2 );

It’s more than that, but it’s the first thing the parser saw. None of this from the last line belongs: , 10, 2 ); The last line should only be }

You’re mixing two styles of filter hooks. Either is valid, but the styles cannot be mixed. First style, named callback:

add_filter{'hook_name', 'callback_name', 10, 2 );
function callback_name( $data, $more ) {
  //do something
  return $data;
}

Second style, anonymous callback (or “closure”):

add_filter('hook_name', function( $data, $more ) {
  //do something
  return $data;
}, 10, 2 );