keyboard
I have Russian/English stickers on top of the keyboard, which in the Russian layout works for almost every key I have tried, but not the "?" and "/" key, which in the Russian layout should be "." and ",". As a result, I do not know (apart from switching to the U.S. keyboard to enter an appropriate keystroke) how to give a "," or "."
How, from the Russian keyboard, can/should I enter a "," or "."?
Yes, should be possible. You just need to unify your keyboard shortcuts in each OS.
For OS X it's in "System Preferences" → "Keyboard"/"Keyboard Shortcuts" for Snowleopard it's in "Keyboard & Text Input" on the left side and then you enter your desired shortcut by clicking on the assigned shortcut and pressing the combination you want.
Note that cmd+space is usually assigned to Spotlight search, so you would need to change that too.
And I think you know how to change it in Windows and Ubuntu, or you're able to find out - right ?
This bug was fixed by Apple with update 10.9.1 of 12/17/2013
http://m10lmac.blogspot.com/2013/12/mavericks-update-fixes-japanese-input.html
Best Answer
To have your Russian layout conform to what you are used to, go to system preferences > keyboard > input sources and use the + and - buttons to replace "Russian" by "Russian PC". The latter puts period/comma at the bottom right corner.