Yeah... Our ability to search posts in Russian is basically broken. It's at least in part a side-effect of our limited support of non-English interfaces, encodings, etc. We're working on improving our support for localized sites, but it's still gonna take a while.
Unfortunately, neither of the solutions proposed here are going to work.
- You can only have 5 tags per question. Make them count. Each tag is supposed to help categorize that question and help others find it later. Even if there wasn't a limit on how many tags you could add, adding tags that aren't actually tags would go against the philosophy behind the entire system.
- Tags apply to questions, not answers. Adding tags to a question based on something that is actually an answer can be confusing.
- Right now we lack support for non-English tags. It's a work in progress...
- How would this work, anyway? Would you attempt adding a tag for every possible form of every possible related term?
The list of problems with posting a separate answer is shorter: it's not an answer. Therefore it doesn't belong on the page as one. Doing otherwise would be basically misusing the platform.
Neither of these proposed solutions also solve the basic problem of "who will be maintaining these?". Neither solution will scale with an increasing volume of questions. Your new users will not know they're supposed to tag things a certain way, for example, so someone will have to police this constantly and update tags/answers whenever something changes. This is not sustainable. At best, you will just end up with a bunch of broken windows all over - posts that were partially processed and updated to follow a particular format.
We at Stack Exchange are working on the localization and search issues. Hopefully we'll be able to update the "related questions" and "possible duplicate" checks to work with the cyrillic alphabet.
In the meantime, I suggest you guys rely on Google searches to do the right thing. Most of your traffic will (or should) be from Google anyway, and they're the search experts. They got the whole nominative vs every other case thing figured out.
I also like Alenanno's suggestion of offering translations in the answer where possible. In the long run, though, I think search tools will catch up.