When you create a new Windows Form, it does not have have *.resx file and the related code in the designer.cs file. When you set the Form's Localizable property to True, VS adds the following code to the designer.cs but it also then generates and adds the *.resx file.
System.ComponentModel.ComponentResourceManager resources = new System.ComponentModel.ComponentResourceManager(typeof(Form2));
this.SuspendLayout();
//
// Form2
//
resources.ApplyResources(this, "$this");
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
this.Name = "Form2";
this.ResumeLayout(false);
Since VS adds the *.resx file, there is no way to find and replace or cut and paste only code.
I tried to record a VS Macro to automate it however, it would not record changing the Localizable property to True (not sure why)
This temp macro maybe a start for you. You could write get the list of your form filenames and loop through them with the macro code below.
DTE.Windows.Item(Constants.vsWindowKindSolutionExplorer).Activate()
DTE.ActiveWindow.Object.GetItem("WindowsFormsApplication1\WindowsFormsApplication1\Form3.cs").Select(vsUISelectionType.vsUISelectionTypeSelect)
DTE.ActiveWindow.Object.DoDefaultAction()
DTE.Windows.Item(Constants.vsWindowKindProperties).Activate()