You see, when God’s judgement came upon Israel from the Babylonians, the prophets all said the nation would know that judgment has come because they will hear “a nation whose language you do not know, nor can you understand what they say (Jer. 5:15) and “unintelligible speech which no one comprehends, of a stammering tongue which no one understands” (Isa. 33:19) and “through stammering lips and a foreign tongue (Isa. 28:11). That final verse is quoted by Paul in the famous “tongues chapter” in 1 Corinthians 14. Therefore, in the same way, people speaking foreign languages (Acts 2, 10, 19) was a sign to the Jews that judgment was once again upon the nation.