My bad. I thought the oxt was just renamed zip, as usual. (Actually, oxt is already zip.) I should have been more careful.
OK, then, the extension with all locales “en” works on my system. The extension “en-US” works also.
Would the dictionary I installed stay intact, or would it get replaced with the new dictionary? In case 1. Any updates to the language in the update would not be installed on that persons computer. In case 2, the dictionary would be removed and would no longer work.
If the extension is installed in the user profile, it will be replace by the one bundled in LO/OO only if its version number is lower than version extension bundled in LO/OO.
If the extension is shared for several users, I don’t know, I never tried. But I assume it will work the same way.