OT: 'Lingua franca' might be my favourite term. AIUI its a Portuguese term to refer to the 'Frankish language', the Franks being Germanic (not Gallic; somewhat akin to the situation of Angles and Britons) and somehow it's used normally to refer to English [not exclusively ofc, there are many lingua franca].

We could just say 'common tongue', but that's just not the English [Language] way - far too simple.