Anything sung in the praise of the Lord is Śruti-mantra. There are songs of Ṭhākura Narottama dāsa, one of the ācāryas in the Gauḍīya-sampradāya, composed in simple Bengali language. But Ṭhākura Viśvanātha Cakravartī, another very learned ācārya of the same sampradāya, has approved the songs by Ṭhākura Narottama dāsa to be as good as Vedic mantras. And this is so because of the subject matter. The language is immaterial, but the subject matter is important.