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I've seen this once - but it wasn't a test or anything - a lot of uncontrolled variables makes it hard to say it was the link or not.  But, we changed and improved. We had site.com/es/word.   When we changed it to site.com/es/palabra and it started ranking much higher. My guess would be that through co-citation and such, more sites link to /palabra/ type links than they do to English words.  Perhaps the same has happened to you?