How do I translate my Japanese grades when applying to American?
Since your WES score is higher than your English translated Japanese score, I would use the WES score. And in my essays or personal statement, explain that you've used the WES score. Plus, since WES is very reputed and a well-known third party, there isn't likely to be any complaint. As always when in doubt check with the program director of your specific program. They will be able to explain any policies in effect within their department or college.