Hello, I'm wonder what's the correct and diplomatic answer if an employer asks you during the interview ( what's the expecting monthly salary )?

The best answers to any interview questions about salary are simply to respond with “I base my salary expectations off of the exact expectations of my role in the company. What is the salary range you are expecting?” Answering questions like this can only lead to further discussion about pay, which reflects negatively on the interview, and can cause you to overshoot or undershoot the company’s target salary. See if you can get the company to answer this question or make an offer first.

The Diplomatic answer will be - I have mentioned my current package in my application so I would request you to suguest a Package as You will be aware what would company pay me after this Interview.

the most convincing and heart keeping answer is "I AM VERY CONFORTABLE WITH SALARY SET IN YOUR POLICY FOR THIS POSITION" because usually the sallaries of the employees to be hired are already set in apporved proposal.