unknown language \selectlanguage{english}

\usepackage[english,ngerman]{babel}

Then ngerman is active at the begin of the document and you can switch to english. If you cannot change the package, then write before \documentclass:

\PassOptionsToPackage{english}{babel}
\documentclass{whatever}
...

You have to use:

\selectlanguage{English}

With a 'E' it should work.

Work with polyglossia and should with babel too.