I am reading through the NIV looking for what I call “NIVisms,” phrases that stand out as a little unusual. Every translation has them. I suspect that sometimes these NIVisms are the result of hours of debate, and there simply was no easy solution.
In the story of Pentecost, when the plethora (I love that word) of people heard the disciples speaking in tongues as evidence of the newly bestowed Holy Spirit, they were bewildered because “each one heard their own language being spoken” (2:6). Here is an example of our decision to use “their” as an indefinite, referring back to one or more than one. I think this works fine here, even though you have the singular “one,” then “their,” and then the singular “language.”
But when you get to v 11, it gets stranger. The NIV reads, “we hear (ἀκούομεν) them (αὐτῶν) declaring the wonders of God in our own tongues (ταῖς ἡμετέραις γλώσσαις)!” The reason this sounds unusual to me is a confusion of number, but the confusion is in the Greek.