I downloaded and enabled the
admin_language module, and in one of its configuration settings, I enabled
Use administration language for the core toolbar, after which it threw out this error:
Notice: Trying to get property of non-object in _menu_build_tree() (line 1377 of /var/www/html/includes/menu.inc).
The line 1377 is:
$tree_cid = 'links:' . $menu_name . ':tree-data:' . $GLOBALS['language']->language . ':' . hash('sha256', serialize($parameters));
I've done this before in another project and it didn't throw an error.