2 (And when they heard that he spake in the Hebrew tongue to them, they kept the more silence: and he saith,)
Why did they react to the fact that he spoke Hebrew by quieting down even more? Supposedly, by your argument he should have been speaking it all along.
Of course, if you'd started quoting Acts 21 at verse 37 instead of verse 29, we'd easily see the answer: he was speaking Greek first.
That's not the point. The point is that the people despised anything Greek. The reason the people became upset was because Paul brought a group of Greeks that he was ministering to into the Temple.
"...And besides, he has brought Greeks into the temple and defiled this holy place.” - Acts 21:28