What classifies as English? When is a (originally foreign) word English and when isn't it?

I’ve been having an ongoing debate with a very learned friend here about what exactly is English – what is the English language – and more specifically, when is a word English and when is it, say German or French.

I don’t want to include the debate here – and I don’t want to state my point – but rather leave the question as is – and see how you jellies define the English language. Then I will be happy to add my thoughts.

But for now:

What is English?

When is a foreign word (a German word, e,g,) English?

What is the English language authority – and who decides what is English and what isn’t – both for you – and in general – if they differ.

Thank you.