Troubleshooting

Nothing happens at all

Check, in this order: the plugin is enabled, at least one domain row is filled in and saved, Search Engine Friendly URLs are on, and the System - Language Filter plugin is enabled. The settings screen warns about the last two. Then check that the host you are testing is exactly the host you configured, including or excluding www.

The language code is still in the URL

The code only disappears for the language that owns that domain. On the English domain /en/ goes away; /de/ stays, because German lives on the default domain or on its own. If a code you expect to disappear stays, the row for that language is missing or points at another host.

I end up in a redirect loop

Almost always a second redirect outside Joomla that fights this one: a server rule that forces www while the plugin is configured without it, or the other way round. Make the server rule and the domain row agree. Browsers cache permanent redirects, so test in a private window after changing something.

A visitor sees the wrong language on a domain

That is what this plugin prevents: on a configured domain the language comes from the domain and not from a cookie or the browser. If it still happens, another plugin is changing the router order; disable other system plugins one by one to find it.

I see /_dpl/ in a URL

That is the internal marker for a link to another domain. Inside pages, feeds and JSON it is replaced automatically, and a marker URL that is opened anyway redirects to the right domain, so nothing breaks. If you see it in output your own code produced, resolve it with Domainperlanguage::resolveUrl(); see the article about links in other extensions.

robots.txt shows the Joomla default (PRO)

There is a robots.txt file in the root of your site; the web server serves that file itself. Rename or remove it. The plugin warns about it on its settings screen. If there is no file, check that your server sends missing files to index.php.

The language suggestion never appears (PRO)

Walk through these: the setting is on; the page is associated with a page in the other language; Add Alternate Meta Tags is on in the language filter; your browser really asks for that other language first; and you did not close the bar earlier, which is remembered for 30 days in a cookie on that domain. Test in a private window with the browser language changed.

Staying logged in does not work (PRO)

Check that the setting is on, that both domains are in the list, and that the visitor is logged in on the front end - the administrator is never included. Logging out everywhere needs sessions in the database with session metadata on; the health check tells you whether that is the case. A user with multi-factor authentication is asked for the second factor on the other domain, which is intended.

The health check says a domain is not reachable from the server (PRO)

Some hosting setups cannot reach their own public address. Open the link in the message in your own browser: a 64-character code means the domain does reach this site and only the server cannot see it. Your visitors are not affected. Ask your host about hairpin NAT or split DNS if you want the check to go green.

Still stuck

Run the domain health check and keep its result at hand: it names the domain, the failing check and the reason in one line, which is usually enough to see where the problem sits - DNS, certificate, web server or Joomla.