You have asked me a question which I cannot answer precisely.
I would not be able to define what you understand by “bilingualism.”
There are certainly people living abroad who have become so used to a new language that they not only think but even dream in the idiom of the country. I, personally, have experienced this after a rather long stay in England.
I suddenly caught myself definitely thinking in English.
This has never happened to me with the French language, but I noticed that after a comparatively short stay in France my vocabulary unexpectedly increased.
This was caused not so much by intense reading of French nor by conversation with French people, but was more the influence of the atmosphere—if this expression is permissible.
This is a fact that has been observed quite often.
But once one returns to one’s country these riches generally disappear.
I am absolutely convinced that in many cases a second language can be implanted in this fashion—even at the expense of the original language.
But as one’s memory is not without limit, a bi- or tri-lingual state ends by damaging the scope of one’s vocabulary as well as the greatest potential use of each language. ~Carl Jung, CW 18, Page 789