My four sons speak the languages English, French, and German, and they all got to that tri-lingual state in approximately the same way. That way involved luck and good fortune, and only modest challenges. Take Alex, my oldest stepson.  He was born in Boulder, Colorado and was then whisked off to Konstanz, Germany with his German father, my husband, and his French mother.  From the age of three (and until 18), he was raised in France.  For the most part, his parents spoke to him in their native tongues.  When Alex was 12 years old and had become my stepson, we spent a sabbatical year in the United States, after which he and I spoke to each other in English.  There have been booster shots in the form of classes and travel.  But mostly Alex got the gift of tri-lingual gab from the hard work and complicated lives of his four parents.