Add the language you want to be your Base language first. Uncheck all of the files that Xcode offers to localize for you.
In the Info.plist, change the development region to the language that you want to be your base language. Note that the property is a bit misnamed, because its value should be a language code (with an optional country code), rather than a region or country code.
Close your project in Xcode. In another code editor, open projectname.xcodeproj/project.pbxproj and search for developmentRegion. You should see a line like developmentRegion = English;. Change this to reference the same language you put in your Info.plist file.
Reopen the project in Xcode. Go through all your localizable files and check the boxes next to English to generate the localizable resources. Note that for storyboards and xibs, Xcode might create a storyboard instead of a strings file. If that happens, just change the filetype to strings file.