This is a long-standing issue, see also issues #15355 and #15355. This is not a bug per se, but rather an implementation that needs to be completed.
It is not clear what should happen, when an alternative page language (APL) is DELETED. For instance: Should all translated elements be deleted too? (if yes, the a warning confirmation box about the extent of what is about to happen should probably be issued).
When an APL is HIDDEN, it is not as critical. Maybe elements on the page, which are translations using the hidden APL should be shown with som kind of annotation on their icon, to signify that their APL is hidden..?
Other related issues are #15355 and #15355: Both of these make what seems to be a hack to exclude APL from being shown, but does not address the fundamental problem, namely that several translated elements might have become 'orphans'.
Generally, searching bugs.typo3.org for 'alternative page language' unravels many corner cases of handling languages: It is hard to do this right.