At the installation process some language strings are not found, so the message of installation completed will have a string like this:

MYCOMPONENT_INSTALL_WAS_SUCCESSFULL

The language strings used in the "script.php" file (during the installation process) need to be loaded.
The easiest solution is to just put the language file (for example: "en-GB.com_mycomponent.sys.ini") directly in the ROOT of the zipped component to install.
It will not be copied into the joomla component installed folder, but it will be used during the installation process.

another solution would be to just add those strings (if just few) directly in the "script.php" (I would prefer this last solution).

Fixed.

I removed the translations, but still JText is called for who wants to customize.

I think Cook users are more oriented specific applications for customers than building for the JED. So the more oftenly, installation can stay untranslated because it will be proccessed by an admin.

thanks, I agree with you.