I think lengua means tongue or lenguas means tongues, people speak in different tongues, people speak in different languages they resemble the same thing but context as always can make a difference in understanding and use. You can't poke your idioma out (I don't think)
We know to sing in two languages. We know how to sing in two languages.
Why is it not the first translation and how would you translate the first sentence? With deber in subjunctive?