Not all keys can be used in Visual Studio shortcut keys. You can try it yourself in the Options dialog / Environment / Keyboard tab, in the 'Press shortcut keys' text box. For example, with a french keyboard, you just can't create the CTRL+. shortcut, you can't either create the CTRL+- shortcut (as some non french suggested :-)

Why? Because CTRL+. is just seen as CTRL+SHIFT+; (you have to look at a french keyboard to understand that) because Visual Studio extracts the SHIFT from the key pressed (because it wants to keep it as part of the shortcut), and doesn't remember a '.' character was pressed.

(original image courtesy of AZERTY)

The rule could be something like this:

A Visual Studio shortcut is not recognized or cannot be used on a given keyboard if the part of it that is not a modifier (CTRL, ALT, SHIFT) cannot be created without using a modifier key on this keyboard.

Note: there are some (probably hardcoded?) exceptions to this rule, for example, numbers 0 to 9 are usable without using SHIFT on a french keyboard...