So the biggest thing that you need to work on is your attitude towards this situation. I accept that you're upset because this person was hired against your advice. However, you are now their leader, by your superior's orders, and your duty is to lead this man.
How can you better lead?
Own your feelings and self-talk.
You do something you're entitled to do, but which your coworker does not like. Maybe it's as simple as saying "hey, we're not going to use Haskell for this project, I do not want to hear any more debate, this is the end of that discussion." Maybe someone tells you in response, "That's so undemocratic and evil, in some ways I really like you as a boss but you can be really awful too." How are you going to handle your duty to keep leading this person when your personal feelings are affected by this insult?
Nobody can make you feel any particular way. We receive as part of our culture the "wisdom" that our feelings are out of our control, and maybe in some extreme circumstances (e.g. death of family) that is true: but in the more typical and average case, it takes two to tango. You have a very important part of both interpreting and assigning value to these statements, and you determine your internal monologue and what you tell yourself about it. You might for example say "Oh my goodness what's wrong with me, that my subordinate says this?!" or "He must be having an off day, I hope things are all right at home" or even "Sounds like I'm doing something right." I'm not saying any of these are "correct" but just that to be an effective team lead, you must recognize that you have this freedom.
If you do not choose the "I am outraged that this person would say that!" self-talk, you can have a much more mature discussion about why they feel that way. You can say "I didn't realize that this was so important to you and I wanted to understand what is missing here, did you want the fancy new language because you wanted to satisfy a natural human need for learning new things and being curious and having a chance to explore? Or do you feel like I should have delegated the decision to you because you are closer to the project? Or what?" and then invite them to share, not why you should change your mind, but why they were so upset about your decisiveness in making this choice. And then once they know that you've heard their basic needs, you can respond with your own, "I understand all of that and I'm deeply sympathetic, I don't like my boss making these decisions for me either, but I also have an overriding company-need, which is that we need to be able to keep this software development going if you get struck by lightning tomorrow. It's not a hugely likely circumstance but I tend to be a bit risk-averse with our tech choices and I want that to extend to this project, too. Are there other ways I can help you keep learning new things, other than by this choice of new programming language?"
Make particular observations.
This also gets down to being mindful of the language we use to describe things to ourselves. If you think about anthropologists and doctors and other sciencey folk who have to watch people, they often talk in these "clinical descriptions" where they say what literally happened, trying as much as possible to not interpret it one way or another or give a false generalization. So you don't say "He's so irresponsible, he never shows up on time!" but "This week when I looked out at 9:00am I only saw him at his desk once, typically he walks in around 9:05 - 9:10." This is actually a very powerful tool for a leader to master. It's, first off, very important for you to talk to yourself with these terms, because it will cover your ass some day when you would have otherwise blown up at a colleague over a misunderstanding. You can just imagine the situation:
You: "Pat, where is the billing information for Foo? I hate this. You always rearrange these Project Xanadu files and I can never find the one I want! Why can't you just leave things the way they are?!"
Suppose Pat's been working on a different project all along, hasn't touched those things in weeks, and you actually dropped it behind your desk. You owe him/her an apology and some soul-searching. Compare that with saying the same thing as specific observations of specific past actions without any judgment or evaluation about whether those things were good or bad: "Pat, I was wondering if you've done anything with the Xanadu files recently? I know that you've reorganized them a few times in the past, and I had the Weston billing file out recently, and now I cannot find it." No embarrassment.
But it's more than that. Generalizations are very hard to work on, if you're trying to self-correct. If your significant other says "you never listen!" that is much harder to correct than "whenever I ask you a question and you're on your computer, you keep your eyes facing the computer. Sometimes you understand my question completely the first time I ask it, but the last two or three times your reply was 'sorry, could you repeat that?' and I need to repeat my question again." Now you can probably come up with several solutions, like, "Maybe you can ask me for my attention when I'm at my computer, then I may say 'give me just a sec', then I will finish whatever I'm doing and turn and face you and give you my undivided attention."
Your team members will have the same experience. A pure observation of seen behavior without implied judgment or generalization, becomes something that you can act upon. If your speech in a performance review begins "Obviously my first problem is your messiness," that is much more difficult to correct than "I have noticed that you have a lot of notes and papers on your desk, many are no longer related to whatever you're working on right now. That's difficult for me because I want to give you latitude to do whatever helps you work, but I also value a visually uncluttered workplace, it helps us colleagues think clearer." The first one, your teammate is grumbling about you after a while, "my boss is such a neat freak!" and the second one, your teammate in the performance review might say "well it is hard for me to organize, but since these are just my personal notes, do you mind if I just put them in a disorganized folder in my filing cabinet?" or whatever.