Show/Hide hidden files ...

The Finder, or open Finder Window must be front most (selected) before issuing the ⇧⌘+. sequence, and later releases of macOS 10.13.4 or 10.13.5 must be installed. The Finder View selection does not matter. This alternation feature is primarily used to show hidden "dot" files, though it will also reveal objects hidden with the chflags hidden construct. That includes the $HOME/Library folder.

The first paragraph has worked fine for me in both releases of the operating system, and regardless of the nature of the hidden file. I have reviewed the $HOME/Library/Preferences/com.apple.finder.plist file with Xcode, and see no resource setting for the hidden/nohidden state produced by the keyboard shortcut.

In Finder, click ⇧⌘+G to bring up the Go To Folder dialog. Copy/paste the following into it:

~/Library/Caches

In the Caches folder, drag the com.apple.finder folder to your Trash. You can't empty the Trash just yet, so logout, and then log back in. Empty the Trash, and then in Finder, perform the first paragraph again to see if the keyboard shortcut now works for you.

Command+Shift+period does work. What Apple changed is the OS will no longer show you hidden files and folders on the desktop. Everywhere else, but not there.

To use command-shift-period you need to have an Open Finder window, do you?

Actually that doesn't matter as long as you are in Finder.

What happens when you try from another account or boot into Safe Mode?

Use safe mode to isolate issues with your Mac - Apple Support

You've been given a couple of alternate ways to show hidden files but command-shift-period should also work. It does for me running High Sierra.

It might be some corruption in Finder preferences.

Try rebuilding the preferences:

From your Finder "Go" menu > "Go to folder" and enter

~/Library/Preferences/com.apple.finder.plist

Click Go.

Delete the plist file.

Then, restart, or log out and in again.

(You may have to reset some Finder preferences the way you like them)

I am using the French-Canadian keyboard ... I tried both the «.» period on the keyboard and on the keypad.

I tried that layout and it still works for me.

The only other thing I can think of is resetting the shortcuts to default in Keyboard System Prefs. Did you possible set a shortcut to cmd-shift-. ?

Try creating a new user account and test there.

What keyboard input language are you using? I'm not sure why that would have anything to do with it, though.

Are you hitting the . key between , and / ? The . on the extended keyboard (number pad) may not be the same keystroke.

In Finder > View

use ⌘ 3 then ⌘ J

and click the check box "Show Library Folder"

From Finder what directory are you looking in when you try. Change directories and try again

shift command .