Debt servicing is a negative under devalued currency. If you borrow $100 bucks from me except I use sprockets as my currency and today $1=1 sprocket, but tomorrow $2=1 sprocket, you owe me.twice as much interest and debt. China devalueing their currency does a few things. It makes their goods relatively cheaper on the world market which is why many things are made there (good for them). It makes foreign goods more expensive which increases the amount of Chinese goods Chinese people buy. This can be good and bad probably net neutral in the short run, but stifles innovation in the long run. China is a net loaner instead of debtor so it hurts them here as well since countries have to pay less to pay them back in their own currency.
On a side note I think China is entering their great depression. I think they will need to really change a lot of their fiscal and monetary policy to continue to grow.