I'm writing a fictional novel in English and I have a mythical character who speaks a combo of English-Tamazight (80% English). For those who don't know, it's the native language of all North African people who are called Amazigh. It's considered by many historians to be the source of all languages. We can still see it in the ancient caves and rocks of Northern Africa. It can be written from left to right, right to left, and from up to down (they prefer from left to right) Now my issue is with the letters. They use both the original form written with what's called Tifinagh letters which are beautiful, example (ⴻⵜⵀ ⴽⴰⵄⵙⵏⵖ) and the Latin letters (slightly modifier by adding sounds that don't exist in Latin letters, example (neččnin stɣerdeit). I want to use the original Tifinagh letters but I also want the reader to at least hear the sounds of the words in his head, which means that it's best to use the Tamazight based on Latin letters. Then I thought about using both of them in this way: 1 When the mythical character is speaking I'll use Tamazight in its Latin letters form to help the reader hear the sounds in his head. 2 When the main character finds words and passages written in ancient maps, parchments, or engraved into an object... they will be mentioned in Tamazight based on Tifinagh letters. (ⴼⵊⵀⵓ ⴻⵙⴰⵇⴰⴽⵖ) and the main character will read the translation in English for the reader. Ps: Latins called it Berber. It's a great challenge for me. I really appreciate your advice.
I’d be very wary of using non-Latin script in an English-language book. Just having non-English words or phrases can be offputting to readers (I'm guilty of this a lot, I must admit. My novel for my MFA included snippets of dialog in three languages besides English and my current novel has four). I've had readers indicate that whenever they come across non-English they just skip over it—and that was with readers in Los Angeles and text in Spanish which is ubiquitous there. I have a feeling that if they came across something as relatively non-exotic as Greek or Cyrillic, they would just stop reading entirely.
I think for your purposes, what you propose is workable, the hard part is making sure you don't lose your readers. I would recommend having a first reader who doesn't know a word of Tamazight read the manuscript and make sure that it's manageable.
For models of what you're trying to do, I would suggest looking at Anthony Burgess's A Clockwork Orange and Junot Diaz's The Brief Wondrous Life of Oscar Wao for cases where authors managed to mix languages well. On the Burgess, I would add the note that the glossary which appears in the back of most editions was added by Burgess's publisher and was not originally part of the book.
I think this is pretty decent as you have the letters available in your word processor. I would block quote the words written in Tamazight so they stand out from the main text when used and this can be a nice way to foreshadow some stuff (if a Tamazight word comes up, but isn't translated, you can use it so that the people who read the script are given a heads up but not those who don't).
I'd recommend that if there is a conversation in Tamazight almost exclusively, that the double up of the dialog in the language and the translation not happen, but rather you write the translation with some punctuation use to denote that while the reader is reading the dialog in English, in universe it is being spoken in Tamazight and translates to the same English Language idea.
"(What time is it?)," He asked in German.
This is important because different languages have different turns of phrases that mean the same thing when translated. The above example, if written in German, would be rendered as follows:
"Wie viel Uhr ist es?" He asked in German.
Where this is a problem is that while it is asking for the time, the question literally translates too:
"How many clocks is it?" He asked in German.
Which is a rather non-sense statement that an English speaker wouldn't understand right away to mean "what time is it" without prompting (This exact example is used for comedic effect in Casablanca where German refugees fleeing to America practice their English with Rick Blaine and ask in English "How many clocks is it?" much to Rick's confusion. In real life, both German and English speaking spies in both World wars would often deliberately ask questions with the wrong words specifically because many German and English question words sound similar but ask different questions. The English "Where" and German "Wer" are pronounced similarly, but in a rapid fire of questions a English speaker asking "Where is your name?" could easily catch a native German speaking spy, who is tricked into answering with "John Smith" as "Wer" is the German equivalent of "Who". A native English speaker would either be baffled or if a sporting chance to prove he parsed it correctly, would point to "John Smith" in a list of names (sorry so many examples are German... it's what I know.).
I would also recommend somewhere in the book (like an appendex) perhaps showing the Tamazight Letter to the sound with an additional "sounds like a letter in an example word" as English doesn't use accented letters for it's sounds and thus reading your Latin letters in your question does me little good for pronunciation (I have no idea what the accent mark over the c in your example word does to transform the c sound, and I'm pretty sure my pronunciation of the word as "Neck-nin" is not correct). Since you want people to appreciate the sound, it may be best to modify the word's spelling to a correct pronunciation. This is a common in transliteration (translating one alphabet's sounds to another). For example, the Cyrillic Alphabet has 33 letters to the Latin's 26, but that doesn't mean it has more sounds... a transliteration of Russian cyrillic to English Latin alphabet will result in a word that is a faithful Latin alphabet render of the word's pronunciation in Russian, though the letter count might be different (For example, Cyrillic has two different letters for a short I sound and a long I sound (could be a different vowel, it's been a while since I looked at their alphabet) but when those are transliterated into English, they would use the "I" or "Ie" sound. Even among languages that use the same alphabet will have different sounds for the letters (the German pronunciations for the letters "U" and "V" are "Oo" (as in boot) and "Vey" while English pronounces them as "Oh" (as in doe) and "Vee") and sometimes even the same language has regional pronuciations of letters, such as English, where the letters "H" and "Z" are pronounced "Ayche" and "Zee" in the U.S. but "Hayche" and "Zed" by British speakers (the latter is probably due to the ABC song sounding better rhyming Z with V and Me).