I think that Russian translator has gone too far. I can't check now the translation I own, but there are so many different versions of LotR in Russia that I wouldn't be surprised if another Sil looked completely differently.
Basically, we are translating the
English text, just as Tolkien was 'translating' it from
Westron (See LotR Appendices), so elvish words, which are unfamiliar for Westron speakers, should be left intact.
Eldar is a word in Quenya, its etymologies have been given above. I think all words like
Quendi, Avari, Sindar should be rendered, if necessary (and it is necessary for languages like Latvian and Lithuanian), but under no circumstances - translated. Just be wary: I htought that
Silvan was an elvish word, until I found out that it comes from Latin
silva (?)
The same pitfall is
Tarn, which is an English word, not elvish.
Have I mentioned "Guide to the Names in the Lord of the Rings"? I don't think there is "eldar" in that list, but in any case, all translators should have it at hand. Only keep in mind that it was intended for translators into Swedish and some other scandinavian language(s)! If you don't know how to find this Guide online, PM me.