English is known well enough. It must be if we consider ourself historians. We simply wouldn't be historians if we didn't. At least that's how it works in these countries. Probably North-american historians don´t need to know the language of the societies they study, they might not need to know anything about such societies except how to obtain their goods. Are Northamerican historians so satysfied with the oficial history writen by their oligarchy that they do not wonder what the native sources migh say?