Looking at strings in the linked apptranslator.org website, a lot of them have ampersands (&) e.g. "&Back" or "&Book View"

What do those mean?

Translations seem to put them in the middle sometimes

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on the other hand, there are languages even with "0 untranslated strings" that have untranslated "unused strings" - what are those about, and is it okay to not have those be tracked?

Windows convention for access keys (as opposed to keyboard shortcuts). E.g. you can choose &File > &Open by pressing Alt-F then O (as an alternative to Ctrl-O), with the requisite keys underlined when you press Alt; you can choose the dialog option labelled &Abort by pressing Alt-A, with the requisite key always underlined in the traditional Win32 toolkit. (By convention, OK and Cancel don’t get access keys, as you can always use Enter and Escape respectively instead.) Not every menu item gets a shortcut, but in a competently designed Windows UI every one gets an accelerator.

(This idea is nice but not without its problems. For those of us who regularly use more than one keyboard layout or even system locale, it doesn’t work all that well.)

Looks like the ampersands appear in menu items to indicate the keyboard shortcut key to navigate to that item.

https://willus.com/mingw/colinp/win32/resources/menu.html

When you press ALT on windows you will see that some characters on menu items are underlined. When you press alt and these letters you can open that menu. To indicate which character to act as that you put & before that.

It's been a while, but that might mean that (in the example above) B is the shortcut key for that menu item.