I ran into the following problem: I switched the keyboard layout of my Mac running OS X 10.9 to use a different keyboard layout (French instead of Dutch). The result was that a number of applications also changed their interface language to French instead of English (!). Which is very annoying.
Removing the French keyboard layout did not solve the issue just like that. For some applications, deleting the preference file solved the issue, but for some it didn’t.
I finally resolved the issue by downloading App Language Chooser from the Mac App Store. This developer tool allows you to “hard code” the language of a specific application. Just drag the application icon into the application window, and specify the language of your choice.