Instead of removing just the uls-preferences key, [[ https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FUniversalLanguageSelector/54c182424ef2740565ef8b92de4b9c2112267f58/resources%2Fjs%2Fext.uls.preferences.js#L66 | you bring down everything ]], including data stored by other extensions (namely Popups).
Steps to reproduce:
1) Open a private window (to make sure there are no cookies) and vgo to https://el.wikipedia.org/ (because this wiki has HoverCards enabled by default).
2) Hover an article link and click the gear icon inside the popup.
3) Select the second option (Απενεργοποίηση προεπισκόπησης, that's greek for Disable preview) and click Αοιθήκευση (Save).
4) At this point you are presented with instructiona how to enable preview again, and it is the right time to open the web console of your browser and type `$.jStorage.get("mwe-popups-enabled")` and verify that the preference is saved as `false`
5) Press Εντάξει (greek for OK), the page realoads and now `$.jStorage.get("mwe-popups-enabled")` returns `null` Also try `localStorage.jStorage` to vrify that the complete store is wiped.
6) Hover an article link. A popup appears although it was disabled.