It's not possible to hide menu items for certain languages. I tested both https://www.drupal.org/project/menu_multilingual → and https://www.drupal.org/project/menu_manipulator → modules which work for all menu blocks, but Drupal 8 MegaMenu seems to ignore them.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Any update on this issue? It is still a valid problem with the latest version 8.x-1.6
For modules that provide manipulator, you can implement hook_megamenu_manipulators_alter() to add those manipulator(s) to megamenu result, as showing in we_megamenu.api.php
We do not have plan to directly support those modules at the moment.