A New Language
Question: My book idea is fantasy and takes place in our world so most everything will be in English but I keep wanting the demons and monsters to have their own language. And most of the people wouldn't understand the language but I wanted one of my characters to. So would I have to actually create words and things from that language or can I just have the character understand then translate? I just don't know how to create a language I'm not good at it. Also 2 of the characters are assassins/spy type characters and was thinking they should have their own way of communicating and just recently I thought instead of giving them another language I could just make them talk backwards because most others wouldn't understand them but they would have been trained to learn how to speak that way so they could have full length conversations no problem. Would that be okay? Would it make sense? I wouldn't do full paragraphs in reverse it would just be a sentence or 2 and maybe them speaking normally then slip into it if someone was approaching them.
My feeling about invented languages is that a little bit can add authenticity, mystery, etc. but too much can frustrate many readers. If your readers have to work hard to learn/translate a foreign language, some may just give up on the story.
Sometimes point of view can solve many of the challenges. The main character's perspective is the reader's perspective, and you wouldn't allow a language barrier to keep your main character out of the story.
For instance, if your MC encounters people speaking a language he doesn't know at first, that can be useful in creating a mystery. As the MC is drawn into solving the mystery, so is the reader. But at some point, the language will be translated,
or the characters will start speaking English, or the MC will quickly learn to understand them, because otherwise the story cannot progress. It will be halted by frustration for both MC and reader.
Having a third character translate can work for brief passages, but can feel awkward if the foreign-speaker is an important character, since the language barrier would prevent direct interaction.
If the MC learns the foreign language, then you will simply provide the conversations in English and make clear that they are speaking the foreign tongue.
As for creating a language, so you can provide samples...
I rather like your idea of backwards speech, because it will seem mysterious at first, but a reader who wants to figure out what is being said can take the trouble to work it out. (Incidentally, Michael J. Anderson, the actor who played The Man From Another Place on the TV series Twin Peaks
said he once taught himself to speak backwards and used it as a secret language with his friends. In came in handy in the series.)
However, there are many other tricks you could use -- from a simple letter substitution, to straight out "making words up." Some writers will imitate the grammar rules of an existing language other than English. How far you want to go with this depends on your interests and how much it adds to the story.
Again, if this is supposed to be a mystery to the MC character, and it's only a few sentences, that's probably okay. Once the MC learns the language, the dialogue should be in English.
If all else fails and you're not comfortable making up words from another language, you can sometimes simply explain that the MC overhears characters speaking a language he doesn't recognize. You might even describe how the language sounds to him without quoting it.
Best of luck.