In the original comic books and animated series, Shredder's real name is Oroku Saki, and he is of Japanese descent. There were rumors that casting a Japanese actor for Shredder may have hurt the project's box office prospects in the all-important and rapidly-growing Chinese market. You need only look at how closely Michael Bay is working with China on Transformers 4 to know that this is 'based' in fact. It doesn't mean Japanese actors are no longer welcome in Hollywood. The Wolverine has a mostly Japanese cast. It's solely on projects like Transformers 4 and Teenage Mutant Ninja Turtles, where China is a co-producer or financier. We saw this with Iron Man 3 earlier in the summer, which had scenes shot specifically for the Chinese audience that weren't seen anywhere else.
When asked if Eric Sachs is actually Japanese, William Fichtner would not reveal any specifics.
An early, leaked draft of the screenplay last year revealed that this version of Shredder was a former military officer named Colonel Schrader, but that could have very easily been changed since then. Jonathan Liebesman (Battle: Los Angeles, Wrath of the Titans) is directing Teenage Mutant Ninja Turtles, which is currently in production.
Teenage Mutant Ninja Turtles was released August 8th, 2014 and stars Megan Fox, Will Arnett, William Fichtner, Alan Ritchson, Noel Fisher, Pete Ploszek, Johnny Knoxville, Jeremy Howard. The film is directed by Jonathan Liebesman.