Use of conlangs
Question: In the interest of being the best writer I can be... and a burning drive to not be outdone by Tolkien... I'm making constructed languages for my fictions. I'm going for complete and speakable languages, not just some code words or nonsense sounds. It's proving really challenging, but I saw that coming. My question is, once I have them ready, how much use is too much use in my story? Authors obviously work hard on their stories, and conlangers work hard on their languages. I'm a person that enjoys showing off my hard work. But can I have too much of a good thing? And how much translation should I provide? I mean, for example, there are a few scenes where a certain pair of characters are conversing in their native tongue simply for the pleasure of excluding the third character (a foreigner) from the conversation, but should the reader know what they're saying? I guess in short, I'm asking this: Once I have fictional languages ready for my fictional world, how would you suggest I use them? Much appreciated!
I would suggest you use other languages sparingly (as Tolkein did). A few words and phrases may be enough to establish the language's existence, but you don't want to make too much work for the reader. Readers don't want to have to learn a new language to enjoy the story.
The same applies for having characters converse in their native language. What's the point of including a long dialogue the reader won't understand? Even if it's not essential to the plot, after a line or two it becomes boring. You just want a little bit of the language to create an authentic feel. Switch back to English as quickly as possible or simply say that the characters spoke in their own language which the main character does not understand.
Translate words unless their meaning is obvious or you're trying to create a mystery (as in a puzzle that requires translation to solve). That's another reason to use the languages sparingly. Too much translating also makes the plot drag.