No plus or minus button in Xcode localization settings
I just meet with this problem and I was using xcode 4.5 preview 3. Couldn't believe apple have made this kind of mistakes and not fixed it even when its at preview 3. Therefore I believe they just have changed the way localisation work so I poke around the UI until I finally found it.
They have shifted it. You first have to add the language you want to the project first. Then after it will appear as a check box item in the UI you show. To add the language you need to click go to Project -> Info then click "+" to add new localizations.
Hopes that helps.
The buttons scroll out of view when the "+" is used to extend the list. Use the mouse to grab the bar right below the list of localizations and drag it down to make more room, and the buttons will be visible again. Then you can click on the "+" to see a menu of additional localizations.