Whats the best way to keep one language while learning a similar one?
The best way (only way?) not to forget languages is to speak them with native people for a few years. This is possible only if you live in a cosmopolitan environment where you naturally bump into people who can speak only one of the languages that you are trying to develop and not the other ones. This is the reason why we find street vendors in countries who travel between the borders of the regions where diverse languages are spoken, to be reasonably fluent in many languages at the same time. This is because the guy wants to make a sale and he can do it better if he can make a conversation in your native language. He may also end up meeting folks that can speak only their tongue and nothing else and hence the vendor ends up picking up that language. For example if you happen to be travelling between Germany, Switzerland & France borders then you may end up speaking German and French and still retain both. Bottomline is you would have to speak the language with people who can understan
