With deflation, interest rates might be low [0], but they aren't likely to decline over time (unless the rate of deflation is increasing), whereas salaries will decline over time. So the interest rate won't drop along with your salary. Your salary will drop, and while the interest rate might (with the caveat noted previously) be low, its not likely to go down over time (if it does do so constantly due directly to deflation, that means your salary is likely not only declining, but doing so at an accelerating rate), your salary will be dropping both in nominal terms and proportional to the interest payments on your debt.
[0] but probably not; availability of credit will be low because risk-free instruments -- cash -- with a positive expected real rate of return exist, so there is little incentive to lend. Low credit availability doesn't make low interest particularly likely (it does make high interest rate volatility more likely, though.)
So in the EU interest rates are pegged to salary? Can it go negative? Because that's what needs to happen for loans under deflation to make sense. Your effective interest rate in a deflationary economy is deflation rate + interest rate. If you take out a 2% loan with 2% deflation your effectively paying 4% rates. The higher the deflation, the worse your rates become.
Now, if interest can go negative, then borrowing under deflation makes sense. But lending doesn't. Because the lender would be handing over money that would be more valuable kept under a mattress than loaned out at a negative interest rate.