Good news for Cymraeg lovers, there are two excellent Welsh dictionary apps out there that you may want to check out: "Ap Geiriaduron Cymraeg/Welsh" and the "GPC Geiriadur Welsh dictionary".
The first one is a very handy (and free!) Welsh-English-English-Welsh dictionary that should work on both Android (from 4.0 and up) and iOS phones (from 7.1 and up). The second one is actually a Welsh-Welsh dictionary, very thorough, too, (if, like me, you're into etymology hunting, you'll love it) and it's also free. That last one is still sadly absent on the Apple store, but it should work on Android phones (from 2.2 and up).
Here are the links if you'd like to find out more: