Merriam Webster Dictionary— Speaking in Tongues:
To say strange words that no one can understand, especially as part of a religious experience.

It is essential to comprehend the definition of the word "tongues" as translated from the original Greek New Testament manuscripts.
Both "dialektos" and "glossa" in Greek were used interchangeably for the translation of "tongue."
Thayer's Greek Lexicon provides explanations for both terms.

1. (A) "Glossa" refers to the tongue as a member of the body, the organ of speech (Mark 7:33, 35; Luke 1:64; Luke 16:24; 1 Corinthians 14:9; James 1:26; James 3:5, 6, 8; 1 Peter 3:10; 1 John 3:18).
(B) It also means the language used by a particular people in distinction from that of other nations (Acts 2:11), as seen in later Jewish usage (Isaiah 66:18; Daniel 3:4; Daniel 5:19).

2. "Dialektos" denotes the tongue or language peculiar to any people (Acts 1:19; Acts 2:6, 8; Acts 21:40; Acts 22:2; Acts 26:14).

Glossa and Dialektos simply means an understandable language - a human language that is spoken. It never refers to unintelligible speech.
In Acts 2:9-11, it is used to refer to languages from 15 ethnic regions that are represented.

Some claim that "speaking in tongues" is a language few to none on earth can understand. However, the nouns "dialektos" and "glossa" help illustrate that "tongues" were known languages, referring to real human languages. The synonymous use of these terms can be seen in Acts 2:4-11, where foreign Jews hear the Apostles speaking to them in their own native language.

Acts 2:4 And they were all filled with the Holy Ghost, and began to speak with other tongues [glossa].
In Acts 2:5-6, the crowd was bewildered because each one was hearing them speak in his own language, and they were amazed because those speaking to them in their own native languages were "Galileans."
Acts 2:6 …every man heard them speak in his own language [dialektos].
Acts 2:8 And how hear we every man in our own tongue [dialektos], wherein we were born?
Acts 2:11 …we do hear them speak in our tongues [glossa] the wonderful works of Yahweh.

What is called tongue speaking in Pentecostal, Charismatic and Neo-Charismatic circles is one of the biggest shams and greatest follies in the 2,000 years of Christian history.
Similar concepts of “speaking in tongues” can also be observed in Sufi practices in Islam, where extreme claims have been contested by Sunni Muslims regarding their validity and authority.
It is known as Shath.

If you listen to the people that speak in tongues, you can hear a subtle chuckle, they are amused by it too .