A short answer would be that the two languages are just too different to be translated into each other mechanically. Some might even say fundamentally different. As a native speaker of mandarin who is also fluent in English I can assure you translating between the two is really hard, when I switch to English mode I construct sentences and use expressions almost completely differently. And as someone already mentioned, you need to have a really deep understanding of the culture behind a language to successfully understand all the subtle things in it in order to translate.
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