On a recent install of 10.10 I can't toggle with Alt + Shift between USA/Isr, even though I can toggle by pressing the language icon on the tool bar.
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if you are using Ubuntu 14,the keyboard shortcut is Super (Windows Logo) + Space bar,it will toggle between the languages.
if you have more than two languages,shortcut for the next source is Super (Windows Logo) + Space bar and for the previous source is Shift + Super (Windows Logo) + Space bar.
you can change the shortcuts using text entry setting.
Related to the feature you mentioned, there's currently an open issue: https://bugs.launchpad.net/ubuntu/+source/control-center/+bug/36812
Enabling the Alt+Shift key to toggle between languages will cause that Ctrl+Shift+Tab (used to reverse cycle through open child windows/documents) does not work. At least that's my experience. Is it working for you?