One benefit of learning a foreign language is that you can communicate secretly if the majority of people around you don’t understand that language. This can work if you are certain that no one understands, but it is not advised that you talk about someone in public hoping that they don’t understand. There are plenty of stories online where people have been caught doing this.

Another benefit is being able to silently eavesdrop on people speaking your target language. You might hear someone talking about you or saying something private or embarrassing in public, thinking that no one else understands.

What exactly makes a language effective for secret communication? It is not necessarily the languages that are known to be difficult to English speakers such as Mandarin, Arabic, Korean, etc. You might think that those languages are good secret languages, but they actually aren’t. While most people around you might not understand, it is highly likely that at least one person will know what you’re saying.

If you really want to ensure that your language is secret it should follow a certain criteria:

Few native speakers

Large phonology sets

Complex grammar

Language isolate

No writing system

Factors to Consider

There are multiple factors to consider. Your target language doesn’t have to fit all of them to be secret, but satisfying most of the criteria will help.

Few native speakers: You want a language with at most a few million speakers worldwide. It is better if there are fewer than 1,000,000 globally, since you are less likely to encounter a stranger who understands it well just by growing up with it. Another factor to consider is the number of learners. A quick Google search on resources for that language will give you an idea how many people are studying your language.

Difficult phonology: It will be much harder for someone to listen on your conversations if there are so many distinct vowels and consonants. Have you ever met an English learner who couldn’t distinguish words such as ship and sheep apart? If you have, the problem of listening comprehension will be magnified for a secret language. An example would be Ubykh, a language spoken in the Caucasus, which has 84 consonants (English has only 24 consonant sounds). If the sound for “k” had multiple variants, you wouldn’t know which one it is.

Learning pronunciation will require significant effort, where you really focus on pronunciation. You will have to produce a large number of unfamiliar sounds without mixing them up.

Difficult Grammar: It is hard to get the exact meaning because the grammar has so many inflections. Deciphering it in real time is nearly impossible if you can’t distinguish all those prefixes and suffixes. Exceptions to the rule only make it more challenging as you would have to remember all those exceptions along with the normal rules. If you want a real challenge, you can look into polysynthetic languages, where each word is composed of many morphemes. Most official languages in the world don’t fit this definition, but Native American languages do.

An example would be the Yupik word tuntussuqatarniksaitengqiggtuq which means “He had not yet said again that he was going to hunt reindeer.” Most of these word parts cannot appear in isolation, which means you must remember them all at once.

Language Isolate: The secret language should be unrelated to other languages. This prevents native speakers or learners of other languages from being able to rely on mutual intelligibility without actually learning the language.

A common example of mutual intelligibility would be Slavic languages, where Russian speakers could understand Ukrainian, Belorussian, etc. partially to various degrees or Romance languages, where Spanish speakers could understand Portuguese, Italian, etc..

Language isolates would include Basque, Ainu, etc. In the novel Shibumi, the protagonist learns Basque as a secret language.

The only exception would be if all the related languages were secret languages as well.

For you, this means that you will encounter mostly alien vocabulary. Anyone learning this language would struggle, for the language is related to none.

No Writing System: Many dialects, endangered languages, and minority languages don’t have a written form. This means that it will be much harder for someone to learn, since there is no dictionaries available. It would also be hard to obtain an advanced level of the language without having grown up with it.

One of the hardest languages would be Sentinelese, since you would literally die trying to learn the language. The island of Sentinel is hostile towards outsiders so there is very little chance that you will pick up the language without risking your life.

Conclusion

It is highly unlikely that the languages you know would be considered secret. If your language satisfies at least 2 of the above points, it is a good secret languages.