Sentence:  Informed by the experience of other parents who had successfully raised their children with more than one language and our own observations, we knew clearly that Léandre and Dominique's mother tongue and <WSD>father tongue</WSD> would not have a chance without deliberate 'control' of their linguistic environment. 
Meaning of the word marked with <WSD> tags:  The language spoken by the father, when it differs from that spoken by the mother.