I don't think one can forget every single word in a foreign language one has had a good command of. The quality of my English has deteriorated drastically since I moved to Germany last year, but can't believe I wouldn't be able to speak any English in 10 years, even if I stopped using it starting today. On a slightly unrelated side note: I was shocked when a German patient with dementia who couldn't even recognize his closest relatives suddenly started talking to me in French. It's a mystery what the mind remembers and what not.