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 <WSD>mother tongue</WSD> and father tongue would not have a chance without deliberate 'control' of their linguistic environment. 
Meaning of the word marked with <WSD> tags:  The language spoken by one's mother, when it differs from that spoken by one's father.