You can define and create custom metadata binding handler in order to bind the metadata fields to the external taxonomy system.
Best Practices Following best practices when creating a new handler will help you avoid the common pitfalls.
Creating a metadata binding handler in C# This example shows how to create a custom metadata binding handler to connect the metadata fields to an external metadata source
Adding a new translatable error message Adding a translatable error message allows returning specific errors with localized messages from third-party plugins.