Are there rules on creating a language in-world?

Question

Suppose a creature wants to create their own constructed language for purposes of obscuring communication. Are there any rules that would cover creating and learning such a conlang? For example, suppose my high-INT wizard were to create his own secret language, and wants to teach the language to the rest of the party in order to discourage others from eavesdropping.

• Can the other party members learn the language as they would any other (e.g. through downtime, a feat, etc.)?
• Is a conlang not a "real language" in terms of the rules, and the ability of anyone else to learn it would simply be by DM fiat?

I know that at the end of the day, all rulings are under the control of the DM, but I'm asking whether there exist rules to cover this sort of scenario or whether it is a lacuna. This could affect, for example, whether another creature could use Tongues to understand a conlang - if it is a "real" language, then the answer would probably be yes, if a conlang is simply an informal arrangement (not "really" a true language under the rules), then the answer would probably be no.

Answer 1

You could hack something together with the Linguist feat

The Linguist feat allows a character to create ciphers, per the following stipulations:

You can ably create written ciphers. Others can't decipher a code you create unless you teach them, they succeed on an Intelligence check (DC equal to your Intelligence score + your proficiency bonus), or they use magic to decipher it.

Strictly speaking, the only thing that mechanically distinguishes a Cipher from a language is that learning a language requires an amount of work learning proficiency in it (250 days + 250gp according to the Player's Handbook, or 10 weeks minus 1 week per INT modifier + 25gp per week per Xanathar's Guide to Everything), whereas learning a cipher requires a character "teach" it to someone else, without rules stipulating on how that would work. Also, a cipher can be broken through an intelligence check, whereas a language can't be "brute forced" (RAW) to understand it.

Ciphers are also stipulated as being "written" in this context, but it would be silly to have a written cipher that couldn't also be pronounced as-written, pending some kind of "magical language".

RAW, the process of creating a "proper" new language is not provided by the rules

Your DM will need to come up with something your character will have to do to justify their ability to create this language. Since the act of learning a language takes, at minimum, anywhere from 1/5 to 2/3rds of a year (depending on whether you use the PHB or XGE rules), your DM should probably specify that creating an entire new language would require not less than a year, and probably would take more than that, pending your intelligence score.