I think one example of invented language to consider as we evaluate our options here is Lapine, the language of the rabbits in
Watership Down*. Of course, we are not given enough to decipher a grammar or anything like that - there are two pages of vocabulary words and translation of some names. The entire story is, of course, in English. But particularly 'rabbity' things that pertain to rabbit life and culture are given in Lapine, and used often enough that the reader learns the meaning and understands what is meant.
As a few examples... Rabbits can only count to four. Any number over four is 'a thousand'. It's used to generically mean 'many'. So, the author could keep saying 'a thousand' all the time...or teach us the word
hrair. There is a generic word for all the predators that eat rabbits -
elil. And
-rah is a suffix for the leader. Thus, the trickster rabbit folk hero, El-ahrairah, is 'the Prince with a Thousand Enemies'.
These words can come up in conversation. The rabbits can ask if there is any sign of
elil, rather than asking about particular predators such as foxes. By giving the names for the Sun (Frith) and the Moon (Inlé), we get a 'rabbit' flavor to any time-of-day talk...and also an easy connection when they tell the folk stories about 'Lord Frith' or 'The Black Rabbit of Inlé'.
We could do something similar with our elvish - figure out what words are particular to elven culture, and then use and teach the audience the meaning of these words, which can be sprinkled into English dialogue. An obvious example is
elen 'star'. In season one, we taught the audience the meaning of 'Estel' (Hope) [though I do not think we distinguished it from
Amdir]. But concepts like 'sea-longing'** are obviously peculiarly elvish, so if we used the elvish word for them consistently, it would help to develop our sense of elvish culture. I'm not sure what the best elvish words are, but stuff like tree, star, mountain, hope, king/queen....
I don't know that we'll ever have an excuse to count things out in 12s, or to use elvish names for seasons or days of the week or units of measure, but touches like that can help to build a culture...or make it more remote/foreign for the viewer. It depends how it is handled, and how naturally the ideas can be introduced without being bogged down with explanations. Obviously, this requires that we be both clever and intentional about our use of elvish in the script, which is why I bring it up
.
*My copy of Watership Down is on a different continent, so I apologize for any errors. Here's the complete vocabulary of Lapine:
http://cs.ulm.edu/~pdw/rabbit/lapine.html
**I don't actually know if Tolkien coined this word or not, but in Quenya he did give us
hroafelmë (bodily desires) and
fëafelmë (spiritual desires/emotions), as well as
lengwë, meaning 'desire for what is lost or absent' - a very elvish concept! The Adûnaic word for 'longing' is
zâir.