I didn't know that about kindle, however i'm trying to eliminate devices from the shortlist, not add them.
About the Clara: waterproofing is not a must have, but rather something that might help tilt the balance on otherwise equivalent devices.
I was leaning toward the inkbook prime hd, but now I've read that its predecessor, the non-hd prime, only has polish-english dictionaries and no option to add any others. I can't find out if that's also true for the prime hd.