Steps to reproduce the issue

Related to #27130
Try to install an extension including the language folder and files such as:

language/en-GB/com_xxx.ini
language/en-GB/com_xxx.sys.ini

Use the 'description' tag in the XML installation file:
COM_XXX_INSTALLATION_MESSAGE

Expected result

The string COM_XXX_INSTALLATION_MESSAGE is correctly translated based on the language string loaded through the file language/en-GB/com_xxx.sys.ini

Actual result

The string COM_XXX_INSTALLATION_MESSAGE is not translated

The string is correctly translated if the language file included in the component is named
language/en-GB/en-GB.com_xxx.sys.ini
instead that
language/en-GB/com_xxx.sys.ini

System information (as much as possible)

Joomla 4.0 Beta 1

Additional comments