You can localize the insert Custom Link dropdown by using the Localization property of RadEditor:
RadEditor1.Localization.Tools.InsertCustomLink = "Placeholders
However, it is not possible currently to change the localization string of the Insert Snippet dropdown. This is in our ToDo list for the upcoming hotfixes of RadEditor "Prometheus".
For your convenience I have made and attached an example with a custom dropdown. You can use the solution to implement your dropdown scenario.
the Telerik team