Okay, here is my first thought on how to show the 'evolution' of elvish language. This is just one detail - we'd need a lot more than this to show the audience what is happening. But have to start somewhere....
'A star shines on the hour of our meeting' is an elvish greeting, as we all know. But it is not a greeting from VALINOR, where everyone lives in the light of the Trees and almost never sees the stars. It is clearly a greeting first developed in Middle Earth, but before the Sun and Moon. Let's make it the greeting at Cuivienen. It is polite, poetic...not really a greeting from the road (the Journey), and so fundamental to elvish culture that ALL elves could have a memory of it.
So, sometime during Season 2 episodes 1-3, we hear this greeting (maybe even multiple times). When the elves meet Oromë, is this how they greet him? When the Ambassadors return from Valinor, is this how they are greeted by their people (or their wives)? Anyway, we can certainly work it in once or twice. And we can use PROTO-ELVISH* (with subtitles) to do it.
Then, we show the Journey. When people greet each other on the journey across Middle Earth, we can introduce 'Well met!' instead as a faster, more casual greeting. A scout goes ahead and comes back, and is greeted in this fashion. When Elwë returns to the Teleri, his brother Olwë can greet him this way. And this greeting is in SINDARIN. We can save this for after more time has gone by if we like, but sometime between Season 2 Episode 4 and early in Season 3 this can happen. Preferably multiple times, so the audience is used to hearing it. Hopefully, we can use it enough times that it doesn't even require subtitles any more, but since TV shows are broken into episodes...I imagine we'd use the subtitles anyway, just to help out the viewers.
Now, moving ahead to Season 3, we have the meeting of the Noldor and the Sindar in Middle Earth for the first time after the return of Morgoth. The Sindar start out by greeting 'Mae govannen!'** (
Well met, but without subtitles this one time), while the Noldor say 'Elen síla lúmenn' omentielvo!'
We will have language confusion and misunderstanding in that scene, but the astute observer will realize that the Noldor are *not* saying the same thing that was said back at Cuivienen, even though the subtitles are the same! So...we will be able to actually show the language evolution on film in this way. We cannot rely upon the audience noticing that, but it will be there, to reinforce whatever else we are telling them about the changed language. And what better sentence could we use to sum up elvish culture than this?
We will of course have the opportunity for hobbits to learn this traditional greeting in Quenya many seasons down the road - perhaps we see Bilbo learn it, and then Frodo use it.
* I am not sure how easy that would be to reconstruct. I know that we have the primitive elvish words for star and shine, but I'm not sure about hour or meeting. [Day, year, twilight, and morning are all available, though.] Presumably, someone somewhere has already thought of this. In Sindarin, it might be: "Êl síla erin lû e-govaned vîn." I don't know how much primitive elvish we want to use (if any), but they *did* have a name for Melkor (the Hunter):
Mbelekôro. Other than that - probably just the work
kwendi?
Here is some background on primitive elvish:
http://folk.uib.no/hnohf/primelv.htm
** Technically, that is a familiar greeting, for friends who already know one another. It should probably be a more formal form here, for a meeting with strangers...as these are likely 3rd generation elves who don't know each other, rather than a reunion of old friends. Someone who is better at these languages than I am may finalize the forms used....but perhaps
mae lovannen?