Performance wise it's the same, IMO it's up to your business. For example if your site can sell multiple language version at once - for example, you Swedish site can sell English, Swedish and French versions, and a customer might buy all of them, then it's of course required to have variants instead of languages. 

I'm with Quan on this one.

Since they're different SKUs, I would make each of the book translations be represented by it's own product and variant in epi. You can still use epis language versions of these, but this would be to add market specific descriptions for the book. So that the customer visiting your finnish site can get finnish information about book X's Swedish Translation, while an english visitor will see english information about that same book translation.

Another benefit to this is if there's, for example, a "second print", but this only gets translated into 5 of the 10 originally available languages. You could then add another variant to those 5 products that got a second print.

Regarding searching/filtering. It will always be based on how you implement that specific component. You create the search/filtering implementation based on how you setup your original data. I see no major difficulties in making this component do the things your require, no matter how you decide to model your data. Something like Episerver Find will be able to handle it just fine.

So to summarize my take on this:

Products = One for each translation of a book.

Variants = One for each translation and editions (prints) of a book.

Product/Variant-language versions = Localized presentational data for the Product/Variant for every market you plan to support with it's own localized site.

Note: I include "prints" so that you keep something like this in mind when you make your decision. All of these kinds of "minor" differences to SKUs should be taken into consideration while determining how you should setup your products and variants. I don't know if you'll have support for different prints, but maybe something else that may afffect it?

Hope this was helpful and good luck!