It's really simple: If SE didn't intend for different languages to intermingle, there would be no built-in Auto-Translator, especially with phrases designated just for telling everyone you only speak a certain language, and dictating assignments/instructions in duties. If that bothers such an extremely small minority of people, they have the option of leaving if they feel they are being harassed or "bad mannered" simply because someone doesn't speak their language natively. One could make the argument that because someone doesn't KNOW what to do in the duty finder, yet joins a duty without knowing the duty, this too is fraudulent behavior - again, using the same logic being used for languages.