I tend to offer to switch to English myself, but it highly depends on the situation. I knew quite a few people, who have very good skills of the (not easy to learn) German language and we agreed that we talk English in the UK / US and German in Germany / Austria / Switzerland.

It might be good, if others want to switch to English in a conversation, that you would prefere to practise your German. And you might add, that you do not mind, if they let you know, if you made any worse mistake in use of grammar or words, because some people might find it inpolite to ask,what exactly you mean / talking about resulting in a misunderstanding.