Yes, you've read various translations. But you know, although awkward translations of names and titles are quite jarring at first glance, they aren't that important, because our minds quickly associate a set of letters with a certain image that we've formed. It's just important not to miss details like the significance of surnames among hobbits, so you can't leave Baggins (and Wise-top as well). But translators often mess up dialogues and descriptions. A prime example is Grigoriev's translation, which, as I understand it, was the basis for the voice acting in our version of Lord of the Rings.

How can you change the word Rohan into the word Ristanya?

I was also surprised by this for a long time, until I heard that it was a simple root-based translation. And the word Rochan comes from words related to arenas and so on, but I haven't verified this statement myself. In general, in Muravyov's translation, which I like the most, it's Ristanya, so I'm used to it, and Rohan is mainly associated with the independent Rohlyandia.