I want to extend the backend page layout partial LanguageColumns so that I can add some information in the marked spot (see screenshot snippet). I found that it should be possible according to this link: https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/8.6/Feature-79124-AllowOverwritingOfTemplatePathsInBackendTemplateView.html
However I cannot figure out how/where to add that code. I am using TYPO3 10.4.20
The mentioned feature is only for custom modules and not to be meant to change existing backend modules.
One way which is often used, e.g. in EXT:yoast_seo, is the hook $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['cms/layout/db_layout.php']['drawHeaderHook'] of the PageLayoutController.
You can also manipulate the Toolbar by using the hook $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['Backend\Template\Components\ButtonBar']. For inspiration you can checkout this code https://github.com/georgringer/page_speed/blob/feature/pagemodule/Classes/Hooks/ButtonBarHook.php (only in the branch feature/pagemodule and only a PoC)
