This is a really hard problem. NIRS is already difficult; very weak signal, limited to <2 cm of cortical surface, and haemodynamic response is 5-7 seconds.

I think it's definitely a useful imaging technique for certain (highly specific) tasks. But to detect language? They would need many, many more source/detector pairs, considering the vast swathes of the brain partially responsible. Language is already very fuzzy within fMRI.