Oh, I agree that this would be an extremely daunting and difficult approach! You'd need to hire a language team, not just a translator and dialect coach. I merely point out that films like Apocalypto and The Passion of the Christ exist to demonstrate that making a film entirely in a language that no one in the audience knows (and that the actors do not speak) is not (apparently) a complete dealbreaker.
The more English the better, in general. Tolkien had his orcs speak the Common language, so the hobbits could understand them, thus making the overuse of Black Speech a bit inaccurate! (Well, okay, for orcs from the same clan, maybe, but......)
'Inglorious Basterds' is a mostly English film. (It is also wish-fulfillment revenge fantasy with copious amounts of gratuitous violence, because...Tarantino.) It has several scenes in other languages (French, German and Italian), but even these often switch in to English for portions. I mentioned it as an example because it so often CALLS OUT the language differences between the characters. So, yes, the Americans are terrible (ie, comedic) at attempting to speak another language. They are passed off as Italians in the hope that no one will speak with them, but of course this fails, and the German soldier they are introduced to speaks fluent Italian. They manage 'Buonjorno, Grazzi, Arevidecci' and their names, with terrible accents. A British spy who is fluent in German gives the game up when he uses three fingers rather than a thumb and two fingers to indicate the number three. His accent was suspicious, but passable, but his cultural ignorance gave him away. I learned this distinction on my second day in mainland Europe, so immediately picked up on his error the first time I watched the film. It's that major of a faux pas. The German spy furiously points this out for the benefit of the audience, though. The opening scene is the most significant. It's really impressive cinema. A Nazi soldier visits a French farmhouse, looking for some missing Jews. It is revealed to the audience that the Jewish family (the farmer's neighbors) are hiding under his floorboards. The conversation switches to English, which the farmer understands, but his neighbors do not. The Nazi soldier pressures him to admit that he is hiding the Jews......
I am sure that the production team wanted to lampshade all the American-made WWII films that just have everyone speak English with accents and ignore the language complexities of Europe.
Here is a comment from the actor who played the Nazi soldier - he had scenes in English, German, French and Italian in this film:
As you can see, he was not upset by the multi-language script....but then, he wasn't asked to do Quenya, either.
We will have cases of characters speaking different languages and not understanding one another (I mean, if we preserve *any* of Tolkien's underlying linguistic history). The Shibboleth of Fëanor may be difficult to work in, but Thingol banning the use of Quenya is an essential plot point. We
could use English for Quenya up to that point, with Sindarin in subtitles, and then when Thingol makes his decree, switch to English for Sindarin with Quenya in subtitles. That would likely confuse the audience and fall flat, but we have to do *something!* As for the 'spy' business - during the Tale of Beren and Luthien, we will have several good guys trying to sneak past enemy lines. Finrod and company ultimately fail, but what gives away the fact that they are not actually orcs? Their disguises are good, but....they didn't follow protocol of checking in at Sauron's tower, and in the interrogation, Finrod reveals knowledge of elven realms and ignorance of what is happening in Morgoth's territory (in the Lay, anyway). I don't recall what gives Luthien away in Morgoth's hall.
We can rely on cultural differences beyond language to differentiate different groups. But not making use of language at all, when Tolkien put so much effort into developing that? It seems absurd!
There are sci fi films that have a modest use of invented language with subtitles. Star Wars has it with very little explanation, having major characters not speak an identifiable language or have subtitles (Chewbacca, R2-D2). The audience relies on the responses of those who do understand them (Han, C-3PO) to decipher what they must have said. Jawas and Ewoks speak, but often without translation. Minor characters (such as Jabba the Hutt and Greedo) are entirely subtitled (or translated). And of course, all of the main players speak English.
I think it would be beneficial to recognize what works and what does not, and what would improve a scene or detract from it. I agree that actors are unlikely to do an awesome job of delivering *all* of their lines in a language they have no knowledge of. And I am not suggesting we go 'Attack of the Killer Tomatoes' and have the villains mutter in gibberish sounds with no attempt at conveying meaning. (Even in that, they had a character 'infiltrate' the enemy camp by dressing up as a tomato and learning to speak their language. His lines were subtitled. Sadly, he gave himself away by accidentally blurting out in English, 'Can somebody pass the ketchup?')