According to The Lord of the Rings (author-published of course) the Eldar are the Elves of Aman plus the Sindar only (and returning Noldor of course). The East-elves of Mirkwood and Lorien were not considered Eldar, nor their languages Eldarin (Appendix F).
However Legolas had Sindarin blood in any case. I know The Silmarillion says (a bit) differently, but I'm not sure Tolkien was paying full attention to what he had already published when he wrote that which Christopher Tolkien chose to put into the 1977 Silmarillion.
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