It's a shame there's no standard contextual key one can hold that overlays all the keyboard shortcuts, swipes, clicks, scrolls, etc that are available in the currently displayed UI. Ideally, every window that has hidden UI stuff would have a little indicator in the corner that means "hey, there's something weird you can do here, hold down the 'explain-o-tron 3000 key' to get a hint".
While one can argue that UIs should be immediately understandable, it doesn't seem possible to pull off such a feat in many cases. Keyboard shortcuts are particularly problematic, as an always-visible list would take up too much space in most cases.
What I like about GNOME apps is that they usually have a "Keyboard Shortcuts" entry in their hamburger menu, which shows you all, or at least the most important, shortcuts you can use in the app.
On the other hand it took me a bit to first find this option because it is hidden in the aforementioned hamburger menu.
Android had this "Option" key until Google decided to blindly follow Apple.
Today, an artifact of this feature is still accessible by holding the Back button for a long time, but it is set to simply show the Overflow menu (if the App has one).
I wish so much that Android would have sticked with it. Similarly, many Windows notebooks swap the right Ctrl with a "Context Menu" key. It basically lets you right click with the mouse, but it could have had so much more potential ...
Windows used to be very systematic about putting an underscore on letters that could be used with alt accelerator key; but this seems to have been phased out.
I think the only thing I've seen with a "hold key for instructions on everything" was strategy game Rise of Nations.
It's a thing mostly for languages in which typing a single character takes more than a single keypress, such as Chinese.
Accelerators themselves work for every language, provided that the input method has a way to actually enter the Alt + character combo. So it works fine for e.g. Cyrillic, Greek etc.
While one can argue that UIs should be immediately understandable, it doesn't seem possible to pull off such a feat in many cases. Keyboard shortcuts are particularly problematic, as an always-visible list would take up too much space in most cases.