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I think the issue of translation is largely beside the point here. What you are seeking is the right to publish the book in a new form for a new market, presumably one that the current publisher do...
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#4: Attribution notice removed
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47933 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
I think the issue of translation is largely beside the point here. What you are seeking is the right to publish the book in a new form for a new market, presumably one that the current publisher does not serve. So, for instance, if you wanted to publish this Ukrainian book in Spanish, you would be looking to license the Spanish language publishing rights, either in Spain or worldwide. The fact that you would have to do a translation in order to exercise those rights is orthogonal to that negotiation. What you are after is publication rights for a particular language/territory. Translation is your problem. The next question is, who owns those rights now? The original copyright holder owned them all initially. The publisher of the Ukrainian version may have licensed all rights or only the Ukrainian language rights in the Ukraine. If they author kept the Spanish language rights, then you are going to have to negotiate with them or their agent. If the publisher licensed them, then their willingness to sublicense them to you will depend in part on their contract with the author. Publishing contracts can be structured in many different ways, so I don't think anyone is going to be able to give you one number or percentage. You would probably be better off writing to the publisher and simply asking them if they are interested in sublicensing those rights and see what they say. If they are willing, then ask what kind of terms they are interested in and negotiate from there.