I found the /_locales/ directory in the following location:
~/Library/Caches/com.apple.Safari/Extensions/Evernote Web Clipper.safariextension/_locales
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Seems we're discussing two separate error messages in this thread... I'm not a programmer, so I've not a solid understanding of how they are connected:
#1: the error related to the language localization (original post by joeworkman)
#2: the error related to cross origin requests (added by Tschügge)
For me, duplicating the /en/ directory and renaming /en_US/ seems to work as a hack for removing the console log Errors