Is there a Greek or Hebrew lemma that says the "tongues of angels" are the unknown tongues, or those that people cannot understand?
Though I speak with the tongues of men and of angels, and have not charity, I am become as sounding brass, or a tinkling cymbal. (1 Corinthians 13:1 KJV)
Ἐὰν ταῖς γλώσσαις τῶν ἀνθρώπων λαλῶ καὶ τῶν ἀγγέλων, ἀγάπην δὲ μὴ ἔχω, γέγονα χαλκὸς ἠχῶν ἢ κύμβαλον ἀλαλάζον.