You can certainly use a fictional language to recite your spells, so long as you understand the intent. For some, speaking spells and rituals in a language that is not their mother tongue can help grab the subconscious' attention (as well as languages and words holding the energy of the meanings they carry, if one is also to believe in that).
As far as Elvish spirits in religious and folkish lore go, you'll want to learn older Germanic languages, such as Old Norse or Old English (as Elves are Germanic in origin). My spirits and Gods seem to react more when I am speaking Icelandic and I'm also learning Old Icelandic - for me, speaking those languages raises more energy.
The site I use to learn other languages is called Memrise.com and it is user submitted (and peer reviewed) content. It has most languages, even dead and fictional ones; you'll just have to search for the specific languages you'll want.