I would like to know if any real language has ever been considered so hard to learn that acquiring decent fluency is basically an impossible task. That is, almost no non-native speaker has ever been able to master it sufficiently to take part in a normal conversation.

Now, some languages such as Japanese, Chinese, Russian, etc. are commonly considered to be hard to learn, but it is by no mean an impossible task and each of them has millions or dozens of millions of fluent non-native speakers. What I’m describing is really a language that would be one step beyond, a language with such a difficult grammar/vocabulary that it has stayed limited to native speakers only.

Is there any documented/historical record of such a language?

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    You may find this answer over at Linguistics SE useful: linguistics.stackexchange.com/a/1041/13780 – J. Siebeneichler Oct 28 '16 at 14:52
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    Obviously not, since every native speaker was born a non-native speaker. – Flimzy Oct 28 '16 at 23:12
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    @Flimsy: Not that obviously: Maybe there are languages that are learnable as L1 by little children, but inaccessible as L2 after the critical period. – jknappen Oct 31 '16 at 13:32
  • @jknappen: No, that's a logical impossibility. – Flimzy Nov 2 '16 at 11:09
  • I think the "critical period" is a myth, a self-fulfilling prophecy for those that fall for it. – WGroleau Oct 24 '17 at 18:49

There is at least one natural language (as opposed to the conlang Ithkuil mentioned by michau) that is impossible for non-native speakes to learn: Sentinelese. The reason is that it will probably cost your life.

The Sentinelese language is spoken by the inhabitants of North Sentinel Island, a small island (47.5 km2) that is part of the Andaman and Nicobar Islands.

The Andaman & Nicobar Administration describes the Sentinelese are "very hostile" and that little is known about them. The Government of India's Ministry of Tribal Affairs writes that the Andaman & Nicobar Administration

has adopted an ‘eyes-on and hands-off’ policy to ensure that no poachers enter into the island. A protocol of Circumnavigation of the North Sentinel Island has been made and notified in consultation with Government of India, which have yielded meaningful results.

Forbes wrote in September 2015 that

The North Sentinel island made headlines in 2006 after the tribe murdered two fishermen who had illegally approached the island. After the incident, a 3 mile zone has been imposed around the island, and the Sentinelese have since kept a low profile.

So Sentinelese is impossible for non-natives to learn because the Sentinelese try to kill anyone who tries to access the island. As a consequence, nothing is known for certain about the language, let alone how to learn it.


From the point of view of linguistic characteristics: I have read many articles about the "hardest language" and I have never seen any mention of a language that is too difficult for non-native speakers to learn.

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    That's not really a linguistic trait though... :) But interesting answer just the same! – Flimzy Oct 28 '16 at 23:13
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    This just confirms my hunch that resources are the biggest determinant in how difficult a language is to learn! – Andrew Grimm Jan 16 '17 at 9:18

If we take conlangs into account, Ithkuil is such a language. Wikipedia says: "No person, including Quijada [the creator of the language], is known to be able to speak Ithkuil fluently.". However, we cannot be sure if it is an actual human language, that is, if it is possible to be a native speaker of Ithkuil. As far as I know, nobody has tried to teach it to children yet. It would be interesting.

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