Control with static constructor
It seems that if a control has a static constructor, Multi-Language Add-In adds the language change handler to the static constructor:
namespace MyControls {
    public partial class MyControl : UserControl {
        static MyControl() {
            MLRuntime.MLRuntime.LanguageChanged += new MLRuntime.MLRuntime.LanguageChangedDelegate(ml_UpdateControls);
        }
        public MyControl() {
            InitializeComponent();
        }
...
This will cause compilation to fail because the event handler is not static.