The setup: I'm using Windows 10, 8, 8.1 and 7 with a international keyboard, US keyboard, Windows keyboard, PC keyboard with VNC client TighVNC, RealVNC, UltraVNC to remote control a iMac, Mac mini, MacBook Pro, MacBook Air, running OS X, with Screen Sharing activated as the VNC server.
The problem 1: My keyboard mapping is all wrong, can't input Alt key(Option ⌥ ). It is identified as Command key ⌘ .
The problem 2: My keyboard is international and a lot of the keys are wrongly mapped.
Problem 1) Use RealVNC client. In my experience, alt (Option) is in the Windows Key and Command is in the alt. Check out the documentation here https://www.realvnc.com/docs/mac-keyboard-mapping.html
There is no fix for the alt (Option) key on TightVNC.
I tried to use software on the Mac to remap / change the keys functionality such as Karabiner (previously called KeyboardRemap4Macbook), Seil and even the native system Preferences > Keyboard > Modifier Keys to change ctrl for alt or capslock for alt , but it seems that when on VNC, those configurations are ignored. It's like a different keyboard is being used and it doesn't show up in the Select keyboard menu, only the local one.
Problem 2) VNC for some reason treats your keyboard as it were an US keyboard. You'll have to change in windows, the layout of your keyboard to US international, and by doing this, almost all your keys are going to be mapped correctly in the Mac. You can set different keyboard layout for different applications.
I registered a issue on TightVNC sourceforge account about this, but the project seems to be dead, so no hope. Same problem happens on RealVNC.
For those keys that are still missing / dead a workaround is to use the Character map (while editing a text press Ctrl + alt + space-bar or at the menu select Edit > Emoji & Symbols )
Alternative is to use a different remote desktop solution. If it's for personal use, go for TeamViewer. I tried and it's a lot better, with full support for the keyboard, better connection, imagem, etc, But if it's for business / comercial use, Teamviewer's price is just ridiculous IMO. The cheapest license called 'Business' cost U$ 749.
I can't speak to the international keyboard but I just tried this solution and it worked for a US keyboard. I'm on a PC with a 104-key US keyboard running Windows 10 and TightVNC Viewer into a Mac running El Capitan and stuck on the upgrade to High Sierra with "About a minute remaining" for the last 12 hours and hit Alt-R-L to simulate Command-L and the log opened right up for me:
TL;DR: Opt key is mapped to AltR on US-PC kb and AltGr on non-US PC kb
