Barnes on 'tongues' in I Corinthians 12:8-30
In commenting on I Cor. 12:10, Barnes distinguished "the power of speaking various languages" from "the power of interpreting foreign languages — or of interpreting the language which might be used by the 'prophets' in their communications…. This was evidently a faculty different from the power of speaking a foreign language….
"In an assembly made up of those who spoke different languages, a part might have understood what was uttered" — but the rest, not. So, "it was needful that an interpreter should explain it…. Some had the talent of speaking different languages, or of interpreting…. Others had not!"