Answer: There is no question, historically, and linguistically, that γλῶσσαν and διαλέκτῳ were used consistently to make clear distinctions between Major Language Groups and Language Styles/Dialects, (references below).
γλῶσσαν would have been understood, in that time, to denote a major language group.
διαλέκτῳ would have been understood, in that time, to denote a specific language's style or dialect.
In Acts 21: 40, Luke chose to use, "διαλέκτῳ/dialect" rather than γλώσσαις.
Acts 21:40, Byz - Ἐπιτρέψαντος δὲ αὐτοῦ, ὁ Παῦλος ἑστὼς ἐπὶ τῶν ἀναβαθμῶν κατέσεισεν τῇ χειρὶ τῷ λαῷ· πολλῆς δὲ σιγῆς γενομένης, προσεφώνει τῇ Ἑβραΐδι διαλέκτῳ λέγων,
In juxtaposition, consider Acts 2:11, where "γλώσσαις/tongue/glossa" is used, clearly indicating entire language families:
Acts 2:11, NASB - Cretans and Arabs—we hear them in our own tongues speaking of the mighty deeds of God.”
Acts 2:11, Byz - Κρῆτες καὶ Ἄραβες, ἀκούομεν λαλούντων αὐτῶν ταῖς ἡμετέραις γλώσσαις τὰ μεγαλεῖα τοῦ θεοῦ.
Given the clear distinctions in Acts, the Greek Septuagint, and extant literature--the phrase "Hebrew Dialect," in the New Testament would clearly indicate the Hebraicized form of Aramaic* spoken in Israel.
*Note: IF there had been a Hebraicized form of Greek--it would have pointed to that, (like how Doric is considered a dialect of Greek).
Secular Writers consistently chose "διαλέκτῳ" to distinguish language Styles/Dialects
Instead of using "γλῶσσαν"
It cannot be overstated that "dialect," and "tongue," were very separate things in the Greek Language.
Consider the epic "Burn" and Sarcasm found in Polybius:
Aulus Postumius was mocked--severely--because he was very educated, had a mastery of Greek, but was essentially a "fool" to be laughed at, because he couldn't grasp a specific style/διαλέκτου of Greek --
Polybius 23.12 - τέλος δὲ καὶ ποίημα γράφειν καὶ πραγματικὴν ἱστορίαν ἐνεχείρησεν, ἐν ᾗ διὰ τοῦ προοιμίου παρεκάλει τοὺς ἐντυγχάνοντας συγγνώμην ἔχειν, ἐὰν Ῥωμαῖος ὢν μὴ δύνηται κατακρατεῖν τῆς Ἑλληνικῆς διαλέκτου καὶ τῆς κατὰ τὸν χειρισμὸν οἰκονομίας.
Aulus Postumius deserves some special notice from us here .. Finally he attempted to write a poem and a formal history in Greek, in the preface to which he desired his readers to excuse him if, being a Roman, he could not completely command the Greek idiom or method in the handling of the subject.
(This is a really fun read, check it out!)
Further, Consider Suidas' Uses of the Word διαλέκτῳ, (Dialect):
Sanchuniathon Bio.1 - Περὶ τοῦ Ἐρμοῦ φυσιολογίας, ἥτις μεταφράσθη (namely, by Philon). Πάτρια Τυρίων τῇ Φοινίκων διαλέκτῳ, Αἰγυπτιακὴν Θεολογίαν καὶ ἄλλα τινά.
This use of "Dialect/διαλέκτῳ" does not seem to indicate that the Phoenecian/Greek Dialect, (Punic), is being pointed to, or the actual Phoenician language--until it is realized that it is Suidas who has written that reference, AND uses this same construction in other lists of literary works:
The Idylls of Theocritus - Suidas has a curious note: Θεόκριτος ἔγραψε τὰ καλούμενα βουκοΛικὰ ἔπη Δωρίδι διαλέκτῳ:
English: Doric Dialect
There is no question, here, that Doric is a dialect of Greek, spoken by the Dorians--firmly establishing that the word, "διαλέκτῳ" indicates a "dialect" of a larger language family--(Greek in this case).
The Septuagint chooses to use "γλῶσσαν" to distinguish between major language groups
Instead of using διαλέκτῳ
Dan. 1:4, LXX - νεανίσκους οἷς οὐκ ἔστιν ἐν αὐτοῖς μῶμος καὶ καλοὺς τῇ ὄψει καὶ συνιέντας ἐν πάσῃ σοφίᾳ καὶ γιγνώσκοντας γνῶσιν καὶ διανοουμένους φρόνησιν καὶ οἷς ἐστιν ἰσχὺς ἐν αὐτοῖς ἑστάναι ἐν τῷ οἴκῳ τοῦ βασιλέως καὶ διδάξαι αὐτοὺς γράμματα καὶ γλῶσσαν Χαλδαίων
English: grammar and language/tongue of the Chaldeans
Daniel 4:1 - Ναβουχοδονοσορ ὁ βασιλεὺς πᾶσι τοῖς λαοῖς φυλαῖς καὶ γλώσσαις τοῖς οἰκοῦσιν ἐν πάσῃ τῇ γῇ εἰρήνη ὑμῖν πληθυνθείη
Nebuchadnezzar the king to all the peoples, nations, and men of every language, that live in all the earth: “May your [c]peace abound!
Josephus chooses to use "γλῶσσαν" to distinguish between major language groups
Instead of using διαλέκτῳ
Josephus, J. AJ 3.12.6 - ἀσώσρα καλεῖται κατὰ τὴν Ἑβραίων γλῶσσαν.
it ended in the form of a bell, like common trumpets. Its sound was called in the Hebrew tongue Asosra
Josephus distinguishes between major languages, Egyptian and what is presumably the Chaldean/Hebrew language:
Josephus, J. AJ 2.108 - ταῦτα δ᾽ ἔλεγον πρὸς ἀλλήλους οὐχ ἡγούμενοι τὸν Ἰώσηπον γλώσσης τῆς αὐτῶν συνιέναι. κατήφεια δὲ πάντας εἶχε πρὸς τοὺς Ῥουβήλου λόγους καὶ τῶν πραγμάτων μετάμελος, ὥσπερ οὖν καὶ τῶν ταῦτα ψηφισαμένων, ἐφ᾽ οἷς δίκαιον ἔκρινον τὸν θεὸν κολαζόμενοι.
Thus they, (Joseph's 10 brothers), spake to one another, not imagining that Joseph understood their language
Josephus distinguishes major languages, Greek and Chaldean ... (?) (or Hebrew/Aramaic--either way its a major distinction).
Josephus, J. BJ 1.3 - προυθέμην ἐγὼ τοῖς κατὰ τὴν Ῥωμαίων ἡγεμονίαν Ἑλλάδι γλώσσῃ
I have proposed to myself, for the sake of such as live under the government of the Romans, to translate those books into the Greek tongue, which I formerly composed in the language of our country.