PLEASE CHECK THE UPDATE BELOW.
The solution to this problem is in your Apache configuration. You need to change the regular expression, which turns the paths into parameters for the WordPress index.php file.
WordPress is a very high-level system powered by PHP, which in itself is a very high-level system. WordPress does not have access to URLs as they arrive on the server. It only has an index.php file and it needs requests to access this file so that it can handle the request.
To make this possible, they use mod_rewrite, an Apache module that modifies requests before the server decides what to do with the request.
Check server error logs. You will see the real files that the server was trying to access. Also check out .htaccess WordPress files created in folders. In these files you will see regular expressions that mod_rewrite uses.
I checked your blog and it looks like you have already decided this. I still wanted to write so that others with the same problem could benefit. Especially after you noticed that your mention of another post in another place from two years ago has not been resolved.
UPDATE:
I am glad that I said that "I am almost sure ...": D (referring to the comments on this answer) It seems that things are slightly different from the situation that I decided several years ago.
I checked the WordPress code and did some testing. Initially, I thought that WordPress could not find the article because of / en / however I was wrong. The default behavior of WordPress to search for a message from a URL covers the situation as a positive side effect: let's say I don't have any plugins or any language, so /en/ means nothing to WordPress.
When I want to switch to erik.landvall.se/en/ , it returns 404 as expected. However, if we have a category structure like abc/cde/efg/ and an article like moo , which can be accessed in abc/cde/efg/moo ; when I change part of the link category, WordPress automatically redirects me to abc/cde/efg/moo :
en/abc/cde/efg/moo abc/efg/moo cde/efg/moo ...
all are redirected to abc/cde/efg/moo . I saw the same behavior on your blog except for en /, which brings your plugins to the scene. Now it’s obvious that the plugin handles the url structure differently than WordPress originally does or causes the original behavior to change in a different way.
I wanted to check the source of the plugin, as I promised; but I saw that it is impossible for me now, because it is a product that I must buy in order to see the source, and I do not want to buy something that I will never use :)
Since you said: "In the parent category, I still see the message and go to it without problems." I was thinking of removing subcategories from url. This is a pretty dirty hack; but better than editing the plugin.
We put this ugly php file, fix_lang_subcategory.php in our root folder:
<?php function fix_lang_subcategory() { $p = trim($_SERVER['REDIRECT_URL'], '/'); if (substr_count($p, '/') > 2) { $_SERVER['REQUEST_URI']='/'.substr($p, 0, strpos($p,'/',4)).'/'.substr($p, strrpos($p, '/')+1).'/'.($_SERVER['QUERY_STRING'] ? '?'.$_SERVER['QUERY_STRING'] : ''); } }; fix_lang_subcategory(); require_once('index.php'); ?>
After that, you should see the WordPress 404 file in / fix _lang_subcategory.php successfully. If this works, we can edit our .htaccess file.
At the end of your .htaccess file after
# BEGIN WordPress <IfModule mod_rewrite.c> RewriteEngine On RewriteBase / RewriteRule ^index\.php$ - [L]
add this line:
RewriteRule ^en/ /fix_lang_subcategory.php [L]
What we do here is that we change the entry point with our stupid php file from wordpress index.php so that we can run multiple php lines without touching the source. This only works for the / en / part of the site. Rest works the same as now. In our php file, we check if there is a connection with subcategories, and if so, we change (this is not very good!) REQUEST_URI, as shown below:
/en/maincategory/subcategory1/subcategory2/article/?anything_here_untouched
to
/en/maincategory/article/?anything_here_untouched
then we leave WordPress to work fine.
If the plugin works differently when you include subcategories, this may lead to redirecting the subcategory pages to / en /.
The best decision:
I thought you did not want to use subdomains for alternative languages; but then I saw: "Can you post the version of the translated blog in a subdirectory / subdomain?" according to the comments on your question and in your answer, it seems that the mention of the subdirectory caused a misunderstanding. Since I saw this on the ML plugin website, you can use a subdomain instead of a folder that will save you from all the problems, maybe:
Instead http://erik.landvall.se/en you can use http://en.erik.landvall.se/ ? May be?
Even better solution:
The 90% chance that ML Plugin is causing this, and they should fix it. Running a support ticket can help them realize their problem and fix it for everyone else.