This new feature has just been merged. See #7680

My tests show multiple issues.

1. A bug: the blog sampledata can be installed twice. It stops installing with an error when getting to menus, but it lets create articles and categories with same name and alias as the ones created in the first install. This although we have

if ($el.data('processed')) {
			alert(Joomla.JText._('MOD_SAMPLEDATA_ITEM_ALREADY_PROCESSED'));
			return;
		}

Exemple for categories:

The testing sample data can't be installed twice as we immediately get an error
Step 1 Failed: Another Tag has the same alias (remember it may be a trashed item).
But we should get the alert even before it tries to install imho.

2. Another bug: it lets install another sample data after installing the first one. While Blog has been installed, one can also install the Testing sampledata. Imagine other plugins created and the mess it would create if any of the items has same name and alias in a similar hierarchy.

3. When a site has been created as multilingual at installation time (Setting is in Installation : No sample data, and later one after installing languages, install localised content), it still lets install sampledata from the module, all data set to ALL languages from the ini of the language used in admin at that time.
   Although it does not break anything, it is totally useless on a multingual site and all data added has to be deleted, which is a real pain specially concerning Testing sample data.
   I imagine a possible further improvement would be to create a specific multilingual sample data plugin which would pick the inis from various installed languages and create a more complex multilingual test site, but, in the mean while I suggest to NOT be able to install ini-based sample datas when the language filter is enabled with a specific Alert.

4. Improvements: The Alert displayed before installing the ini-based sample data should be more precise, i.e. stating that it is the data in the administrator language in use which is going to be installed and, if not present, default to en-GB.