I think I've found the issue: I've checked on my Safari (and Chrome) that
navigator.language
returns "en-US", which after the transform turns into "en_US".
The problem is that under "_locales" there is only "en" (and other languages) but no "en_US".
So as far as a quick fix, you guys can either rename/copy "en" into "en_US". That wouldn't require changing the script itself. Of course a more permanent solution would be to fix the script.
Note to the Evernote Web Clip Team: don't ignore this thread. I feel like this is a issue that have been dogging Safari users for a whil