Oh, how lucky you are to speak all the languages you do already. I wish I was at your same level! So, I can not offer advice out out experience, but only personal. My mother-in-laws family spoke Czech, so my vote goes for that, but seriously, whatever you choose, you will be the wiser, learning, yet, another language! I am just wondering, if it is similar, wouldn't it make it a little easier to learn, not harder?