Yes, they're used in English. De facto means 'in reality' or 'in practice', rather than strictly in law. The US dollar is the de facto currency for trading in oil. De jure means 'in law'. De facto is very commonly heard but de jure is quite rare.

They are technically latin, but they are used in English in a legal context. You might hear them being used by lawyers, for example.

Yes, they are, and your best guide is to see if they are included in an all-English dictionary. The answer is that they are. English has thousands words and hundreds of phrases that we have borrowed from other languages. In the case of de jure and de facto, these are Latin phrases, that became part of legal language in English, and were so common in legal contexts that they became ordinary English. Some other examples from Latin include "et cetera," "ad hoc," "alma mater," "non sequitur," and "per diem." There are many other examples from other languages. We use a lot from French, including "bon voyage," "au revoir," "carte blanche," and "grand prix."