Normally, the dropdown menu of the posts filter is a single selection. If you install this plug-in, dropdown menu of “single selection” will change to “multiple choice”.
For example, if you select some of the categories, you will be able to view the post
belonging to any of them in the post list page.
You can send your own language pack to me.
Please contact to me.
jQuery UI MultiSelect Widget
under WordPress 5.5, and uses its own script for WordPress 5.5 and later.This plug-in is not guaranteed though the user of WordPress can freely use this plug-in free of charge regardless of the purpose.
The author must acknowledge the thing that the operation guarantee and the support in this plug-in use are not done at all beforehand.
email to takenori.matsuura[at]gmail.com
twitter @tmatsuur