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We know that using Google translate to auto-translate content can be a form of spam. It also causes issues for GoogleBot in understanding what your page is about.
Google's John Mueller said in a Google Webmaster Help thread that the Polyglot Googlebot has issues understanding it. John explained, "it's important that sites use separate URLs for individual languages."
John said it helps "users who are looking for an English page (even if their browser is set to German), are able to access those pages." It also "allows our polyglot Googlebot to read all language pages and index them separately. If your system currently keeps the same URL and swaps in a translation, that would be a bit confusing to users (and Googlebot)."
So what should you do? John said it would be "better to show a banner ("Click here to see the German version" or similar), and then to go to a separate URL for that language (this could be using pushState, if you're using JS). "
Most multi-lingual sites do implement it this way. But I cannot stress enough, this is important.
Forum discussion at Google Webmaster Help.
