I checked this on a site that was updated from Joomla! 5 to 6, and I have the same problem:

J!6.0.1:

This results in new languages only being able to be installed in an older version.

It is interesting that the German language nevertheless somehow automatically entered the system in a version 6.

Update:

The problem with the German language comes from a faulty update of the language pack:

But its fixed in 6.0.1.2

@woluweb @ahotzler The update site URLs for the translations are updated with an update SQL script when updating from 5.4 to 6.

See here: https://github.com/joomla/joomla-cms/blob/6.0-dev/administrator/components/com_admin/sql/updates/mysql/6.0.0-2025-09-23.sql

Could you

1. Check in your administrator/logs/joomla_update.php log if that SQL script has run.
   There should be a line with text "Ran query from file 6.0.0-2025-09-23. Query text: UPDATE #__update_sites SET location = 'https://update.joomla.org/language." in the log.
   Is that the case?
2. If you have a backup (SQL export would be enough) or a copy of the site from before the update: Was the URL of the translations correct before the update?

If the answers to both questions is yes, then the only remaining question is how it can happen that the URLs for all languages, also the core's en-GB, can be messed up e.g. by a broken language pack, because in the update sites table is a record for each translation.

A language pack should only modify the update URL for its language, not all of them.

@tecpromotion Can you check if the German language pack does that right?

A language pack should only modify the update URL for its language, not all of them.

@tecpromotion Can you check if the German language pack does that right?

I explained this yesterday evening in the CMS Release team.
The German language pack had an incorrect value in 6.0.0.1 and reset the link back to 5. This can happen with any language pack. So it's a global problem. But I'll clarify this with the translators.

Actually I have migrated ~50 websites from J5 to J6 since yesterday morning.
And I did not write down which were the 5 sites having that issue (because I had fixed it manually).
This is silly bc now I don't remember which ones it was, so I can't check the logs 🤦

@tecpromotion And how shall it be fixed for sites which have the problem? With SQL e.g. in phpMyAdmin?

In my opinion, one possible solution would be to include updates for version 6 in the update XML files for version 5 that were distributed via the language pack.

Then, installations with the incorrect update source would receive new packages via this update source that would correct the update source.

I can't say for sure whether this is possible here, but I believe that with extensions, it is possible to mix updates for different target versions.

Another solution could be a quick 6.0.2 release of the CMS which does the same as e.g. https://github.com/joomla/joomla-cms/blob/6.0-dev/administrator/components/com_admin/sql/updates/mysql/6.0.0-2025-09-23.sql for MySQL with a new update SQL script, e.g. 6.0.2-2025-11-27.sql, of course also for PostgreSQL.

Thanks @ahotzler for the reminder

I need some installations where the problem occurs in order to run through the behaviour in different scenarios.
Please contact me by email at stefan.wendhausen@community.joomla.org with further details.