Where should the validation be, in the MVC layer or in the business engine?

A web application backend coded in Spring makes remote procedure calls to a "business engine" which performs intensive calculations.

These intensive calculations have a high error rate mostly because of bad requests from the backend to the engine and/or bad/insufficient data in the DB.

Because of that, it is necessary to provide error codes that can be converted into messages to the user to help identify the problem.

     ----------         -----------          ----------
    |          |       |           |        |          |
    |   USER   | ----> |  BACKEND  | -RPC-> |  ENGINE  |
    |          |       |           |        |          |
     ----------         -----------          ----------
                             |                    |
                             |                    |
                           -----                -----
                   Config |     |     Business |     |
                   DB     |     |     DB       |     |
                           -----                -----

It is my opinion that the validation should occur before making the call to the engine in order to fail fast and avoid waiting for a response that is going to fail inevitably after being sent, queued depending on the load, processed and returned to the backend.

Question: should the validation occur in: A) the backend or B) the business engine?

Bonus question: if the answer was B), should the business engine return the error code and let the backend handle the conversion, or also implement the i18n of the messages?

Additional information:

• The validation logic is no secret and is known both in the backend and the business engine.
• The configuration data used in calling the engine via RPC is owned and stored by the backend (the engine has no access other than the request).
• i18n of the errors is required.

Opinion and/or alternatives for/against any of the approaches is welcome. Thank you.