If you borrow money at 0% NOMINAL RATE, and deflation is 5%, it means the REAL INTEREST RATE is 5%.  Not 0% as you pretended.

Now, if the real interest rate is 5% (which becomes 0% nominal rate at 5% deflation), it would become a nominal interest rate of 10% if there were 5% inflation.

Because nominal interest rate (the one you have to pay) i = i0 (the real interest rate) + p (inflation, or negative if deflation).

In other words, if the market determined a real interest rate of 5%, and there is 5% deflation, the nominal interest rate on a loan will be 0% (you give back what you got) ; if there is 5% inflation, the nominal interest rate will be 10% (you have to give back 10% more than what you got after a year).

The real interest rate is related to the expected average return on investment of capital in the economy (independent of any currency and hence independent of any form of inflation or deflation).

You don't seem to understand that the nominal rate you have to pay on a loan (or that you get on a savings account) includes a correction for inflation from the real interest rate.