Hello Jonas,
the ComboBox template depends on an OnLoadItem slot method to obtain the item contents. Accordingly, when you want the ComboBox to refresh its contents when the language changes, it is necessary to invalidate all items within the ComboBox explicitly so the ComboBox reloads them again. For this purpose you invoke the ComboBox's method InvalidateItems(...) providing in the parameter of the method the range of items to invalidate, e.g.:
ComboBox.InvalidateItems( 0, ComboBox.NoOfItems );
You can place the above invocation within a ReInit method of the GUI component embedding the ComboBox instance. Or, if all ComboBoxes in your application will display multi-lingual contents and all of them should be able to automatically reload their contents when the language is changed: implement the ReInit method directly within the ComboBox class. Then following would be the code of the method:

I just verified the above approach. The MultiLingual attribute on the ComboBox class, or other classes belonging to the ComboBox template will not have any effect in this case. I wonder thus how your implementation worked.
I hope it helps you further.
Best regards
Paul Banach