I really don't think there is a language so hard to learn, because the language You think you're finding it hard to comprehend is someone's mother tongue, spoken fluently. Though I've found some dialect and tongues hard to decode but some have been cute to know. From gestures accompanying sounds and speeches to motions of the eyes, legs and every part of the body to suggest and aid in what has been said. Like the snapping of the finger which is worldwide interpreted as "fast/ in the twinkle of a eye" has made some languages easy to understand and comprehend.

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In our ethnicity alone, which is Yoruba, there are more than 20 dialect spoken locally along the Yoruba tribes. We have Akoko, Ekiti, Ilaje, Apoi, Ibadan, Ibaripa and many more. These tribes has Yoruba as there centralized tongue, it is with this centralized Yoruba tongue that other tribes communicate with each other.
I still find the Russian Language as one of the hardest language in the world though, even for those who are speaking it, they suffer there tongue and teeth to pronounce every word fricatively. Much more like forcing yourself to say some stuffs. It still amaze me how that language is spoken.
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