Its not natural language but can be mysteries in natural language, because "For anyone who speaks in a tongue does not speak to people but to God. Indeed, no one understands them; they utter mysteries by the Spirit." (1Cor 14:2)
"But the one who prophesies speaks to people for their strengthening, encouraging and comfort."(1Cor 14:3) This is the role of prophecy both traditionally and in this verse in the same chapter you quoted from. It suggests that prophecy may not be what you have assumed it to be, so perhaps 'Tongues' fills some role that you have reserved for prophecy.you would also need to find any support in scripture that would suggest prophecy should not be defined as speaking the word of God, but speaking only the words of God which people can conceptually grasp.
In Acts chapter 2 these Jews weren't amazed because they were hearing their native languages (though it was in their native languages). That would not have amazed them, because they were Jews. They were hearing the praises of the LORD, the praises of the unspeakable name, the greatest mystery of all. That was what amazed them. Don't focus on trivial details while overlooking the significance of what happened.Additionally; I can further add that your interptation is not consistent with Acts 2. Here we clearly see that tongues is defined as the function of speaking in a foreign language you don't know or understand, but which the hearers do.
The people were not amazed that they could understand something that would otherwise be theologically too complex for them to grasp - they were amazed that these unlearned fishermen were speaking in their native languages.