TYPO3 Core

I saw that there is a workaround by doing following:

lib.content.element = CONTENT
[...]
lib.content.element.select.languageField = 0

Only then also in the default language content elements with "all languages (value -1)" are displayed.

I don't know if it is really expected that by default CONTENT elements does not display "all language" records in the default language?

All together there are two bugs. The first one is, if you create a content element for all languages (value -1), the backend breaks in typo3_src-9.5.1/typo3/sysext/core/Classes/Site/Entity/Site.php Line 217 - 223 in the method getLanguageById. This one you can fix by the line:
if ($languageId == -1) $languageId = 0;
at the beginning of the method. This bug is only to get the content element editable in the backend. It has no effect to other behaviors.

I can't reproduce this error. In all any installation of Typo3 9.5.1 creating, editing, deleting content elements with language -1 works fine.

The real bug is, that the content element is not displayed in the default language. I don't know any solution for this.

I can confirm this problem

My workaround does not work. The problem with the typoscript command

lib.content.element.select.languageField = 0

is, that it displays all content elements and does not take into account the language anymore. So if you have on another page content elements with correct translations it displays both. But what I want to do is, it should only display content elements with "All language" entries on all languages.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/59993

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60192