I think Japanese would be the most difficult language to learn, because they have more than one "alphabet" and use Chinese characters on top of it!
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I wonder if it would depend on whether you're learning to speak and hear, or to write and read; and whether that matches your learning style? E.g. I thought I was an aural learner, but I now think I must be a visual learner. But I'm not sure at all.
As a kid I taught myself to know and pronounce some Spanish vocabulary from reading a word/phrase list and the pronunciation keys. I then was able to learn Latin and French in formal lessons at secondary school. The Latin helped me to later decode things like appliance instructions from Italy and Spain. I also was able to "catch on" to isolated words and phrases from my sister's German secondary school lesson book and from her tape recorded lessons.
But, as a teacher, and having dabbled in trying to pick up words and phrases in those languages plus Mandarin, Samoan, Fijian, Cook Island Maori, Japanese (with help from son's secondary school lessons), reading phrase.vocab lists as well as hearing them pronounced, and also from having to teach formal English to Vietnamese refugees as well as the Polynesian mix in South Auckland classes and schools in other parts of NZ, I actually think that...
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It's easier for a natural speaker of English to learn another language than vice versa! English has so many exceptions, and is derived from so many lingual roots!
But, formal English studies are fascinating to me anywhoo