Would creating a custom human language be stronger protection than encrypting a known human language? There are some languages that people haven't been able to read (Cretan Hieroglyphics for example).

Does that mean that you can better protect data by making your own language than by encrypting an existing one?

Or are all human languages really just different mappings of symbols to the same or similar ideas meaning that decryption and translation are the same thing?
$$\texttt{NO}$$

In short;

$$\text{We are living in a world where that is ruled by Kerckhoffs's principles}$$

The above may not be totally clear as indicated by Maarten in the comments. The We is the Modern Cryptographers, the World is used for the Cryptography,... In other words, and better;

$$\text{Modern Cryptography keeps to Kerckhoff's principles}$$



Decoding lost languages is not related to cryptography, though some techniques may be related like frequency analyzing. That is people tend to give names to some common objects like the sun, earth, man, women, child, etc.


  Does that mean that you can better protect data by making your own language than by encrypting an existing one?


Go on build one, then;


On day one, one will sell the decoding. Then you need to change your language and then you have to teach it, everybody. In Kerckhoffs's principle, the system relies on the secrecy of the key, not the system. If the key is sold or compromised, we can change the key, not the system. Of course, we assuming that the encryption system is designed to stay against cryptography attacks like AES that stands for almost 20 years and expected to stand more.
The more you talk the more information you will give to decode. You can get the notion from the researcher who goes into amazon forest to communicate with people who had no contact with other humans in centuries.
Actually, you build a large permutation cipher, that is breakable.



  Or are all human languages really just different mappings of symbols to the same or similar ideas meaning that decryption and translation are the same things?


That is not exactly. For example; some languages don't have a name for the color pink or other. Some missing other words for describing things/actions etc where it exists in some languages. Culture, living area affects the language. This is out of the Cryptography context.


  Would creating a custom human language be stronger protection than encrypting a known human language?


NO



Let finish with the Kerckhoffs's principles


The system must be practically, if not mathematically, indecipherable;
It should not require secrecy, and it should not be a problem if it falls into enemy hands;
It must be possible to communicate and remember the key without using written notes, and correspondents must be able to change or modify it at will;
It must be applicable to telegraph communications;
It must be portable, and should not require several persons to handle or operate;
Lastly, given the circumstances in which it is to be used, the system must be easy to use and should not be stressful to use or require its users to know and comply with a long list of rules.