I know this is old, but I also ran into an unexpected error with invalid signatures with Spanish. My skill has both English and Spanish, and all languages use the same request validation logic. Spanish would work most of the time, but one intent would fail sometimes depending on the input from the user. The issue was my encoding. I was using ASCII encoding for one part and should have been using UTF8. So it would fail to validate the signature when the payload contained Spanish with diacritical marks. So, make sure your encoding is correct.