Hi.
Yo’re right, the plugin expects language name to be unique.
I think you might add Simplified Chinese with different language code. I’ve checked such case and there are no problems.
You can not change captions in menu editor, but you could translate pages/categories/posts names.
These names will not be displayed in the right way at admin part ( look here: https://c2n.me/iv70mm ) but they will on a site.