[Just mythoughts]

If you note the 'tongue' spoken was understood, what need would there be to speak in a 'tongue' that nobody understands?

[Always Right]

If you note the 'tongue' spoken was understood, what need would there be to speak in a 'tongue' that nobody understands?

I suppose that speaking in a tongue that is understood is for the benefit of the person who understands, and that the tongue that nobody understands is for the person who speaks. In both cases it is a sign for the non-believer.