Are well designed crypographic codes unbreakable? (NOT ciphers) Codes are not to be confused with ciphers. It is known that all known ciphers are theoretically breakable except the one-time pad and this isn't what the question is about.

Some cryptographic codes are akin to inventing one's own secret language, along with new words, new expressions, and new grammar. I'll refer to these strong codes as "secret languages". Note that when all speakers of an ancient language dies, it becomes a secret language.

It seems to me that cracking secret languages is beyond the realms of cryptanalysis. After extensive search, I haven't found a single secret language that was ever cracked without any known plaintexts such as finding available translations. Are there any ways to crack or even crypto-analyze secret languages only using raw text written in that secret language? It is obviously possible with ciphers because they operate on the syntax level, but secret languages operate on abstractions and high-level meanings.

If the answer is yes, then why is cracking some ancient secret languages is a really hard thing to do unless one finds a translation? I challenge you to crack the The Na'vi language only by reading excerpts written in Na'vi. Also, can you point to one example of a secret language that was cracked only by looking at the text of that language?

If the answer is no, why isn't this used in modern computer cryptography? I presume it's because computers have a hard time manipulating meanings, abstractions and grammar and are better suited for manipulating syntax, letters and bits, and hence only ciphers are practical. But any more info would be appreciated.

A similar question was asked here but the two answers were contradictory, and it was closed, probably because it was focused on ancient languages and was considered a history question.
The answer is "irrelevant", because of a wrong assumption about security.

Today, ciphertext only attacks are not considered any more for real security, because it is much too weak. Known plaintext attacks are the very least to be considered, and that breaks any construction of codes or obscure languages. The reason for this is simple: Laymen might only see the ciphertext/encoded text. But specialists will find ways to get additional information to enable known plaintext or chosen plaintext attacks (for symmetric encryption. Asymmetric has a different playing field)

This is very similar to the argument "security through obscurity", as it is still practiced quite often. Sure, on the first glance it seems a good idea not to give an attacker information about your system. And then later you realize: If that is the only security mechanism, you're screwed when he finds out anyway.

Edit:

In order to avoid further discussions, the answer to


  Are there any ways to crack or even crypto-analyze secret languages only using raw text written in that secret language?


is "No". Under these assumptions (no knowledge about the code), you can't break a single encoded message.

Second question:


  If the answer is no, why isn't this used in modern computer cryptography?


Because your reasoning stops being valid after the first message send or if your assumption is no longer true. Multiple messages start showing correlations, which make single words translatable, then often used statements, then sentences, ...

Additionally, today's cryptography does not consider "ciphertext only" attacks to be a useful argument any more. If you present a new symmetric cipher which is only secure against such attacks, it gets labeled "not secure". Today at the very least, security must hold against known plaintext attacks. And at that point, no classical code can withstand.

Finally, you said in the comments "Codes are just ciphers with a very large key that always gets reused.". This is just wrong. Codes are not ciphers. The two are quite distinct and have their own properties. If anything, you could argue that ciphers are codes, extended with real security. Surely not the other way around.