I don't know what it is that divides the polyglot from us mere mortals, but I don't really understand how people can cope with a working knowledge of multiple languages.
I used to be able to speak German. Not fluently, you understand, but I was quite comfortable with speaking and comprehending German.
These days I believe I can comprehend German almost as well as before. I can still read spoken German aloud and know how to pronounce it. But, I can't "Speak German" to save my life.
This morning, I walked past some Germans speaking German which prompted me to wonder if they could speak English and, if not, if they could speak Chinese. I tried, simply for the fun of it, to produce in my head the German for the phrase "I can understand some German but find speaking German difficult". The phrase I came up with (at the first pass) was "Ich verstehe yi dian dian Deutsch dan shi ich juede spreche Deutsche sehr nan". Hmm. That's not going to fool anyone.
I'm convinced for the non-polyglots amongst us our brains tend to have 'native language' and 'foreign language' sections and that doing something like moving to China having studied German for years at school and evening classes simply leads to the 'foreign' section becoming overwritten. I definitely had some of this problem when I was at school trying to deal with French and German side by side but the virtual non-use of German for the last 4 years (apart from singing along with Rammstein tracks) has left my German ability corrupt and unusable.
The supporting evidence for the 'native' and 'foreign' brain components is also quite ably seen. I know I do this myself but have evidence from others that the act of being 'abroad' switches on the 'foreign' component, even when you don't want it to (e.g. pete saying 谢谢 to people who opened doors for him in Osaka, and basil asking the hotel receptionist about 'le petit dejeuner' when we were in Bologna).
It may well be that this afflicts British people quite badly as we are generally not reknowned for our superior language skills, instead typically relying on other people to be able to speak English but it is jolly irritating.