TYPO3 Core

1. Create a blank TYPO3 Site
2. Configure default language
3. Configure two additional languages
4. Create a page in default language
5. Translate page to the two additional languages
6. Create content in one additional language

Expected: See "Translate" button in the other additional language, offering to translate from the existing content of the other additional language.
Actual: No "Translate" button.

Adding content to the default language will enable the "Translate" button. Editors are able to select the additional language as base for translation.

The value of l18n_cfg doesn't affect the situation.

Use Cases:
Some websites have a lot of languages, sometimes for multiple countries. There editors need to be able to translate from one Language (e.g. French) in one Country (e.g. France) to the same language (French) in another country (e.g. Switzerland).

This is probably closely related, if not a duplicate of #76274

My investigation showed that TYPO3\CMS\Backend\View\BackendLayout\ContentFetcher->getTranslationData() always only checks for default language via hard coded value 0.
The fluid template will use the outcome to determine whether to show the button in EXT:backend/Resources/Private/Partials/PageLayout/LanguageColumns.html.

I guess the logic needs some tweaking to really determine if there is still anything left for translation, respecting connection - free mode, etc. and the possible sources of translations.