Hi,
My screen looks just like that of
michelberdyn (@michelberdyn) above.
I installed this plugin months ago, and it worked as far as this screen goes.
I deactivated it for other reasons (including where to display yet to be decided). I may have gone so far as to uninstall it.
But about a month ago (maybe more), I updated it.
Then thought I’d have another look at where to place it.
It was then that I noticed that it was not displaying all the options for choices of languages (i.e. checkboxes for each language), that it had done before.
I thought this might have been due to the previous version, leaving its settings intact when either deactivated, and/or uninstalled, but then they were getting in the way, or not functioning after the re-activation and/or re-install. This may be caused by a change in the structure of how nthe settings are stored, so the plugins thinks the data is there, but can not access it. Just one possibility.
But I have not updated or changed the browser (nor the whole operating system) between the installation of the functional plugin, and the activation of the problematic plugin.
So this suggests that (as you have referred to) if there is a problem with javascript, then it is due to an alteration of the plugin, and not something inherent in the users system.
The alteration to the plugin may well work just fine on a browser with updated javascript features, but that means the alteration had been made and also that it depends on a more recent version of javascript (even though it originally worked well enough with older versions of javascript).
I hope this information helps in diagnosing the problem.
Thank you