Theodoret

General Comments
"Paul does two things at once: he condemns their ambitious rivalry and teaches them how to use the spiritual gift. Because of the diversity of human languages, preachers received the ability to speak in tongues so that when them came to inhabitants of India, they could use their own language to bring them God's message. Again, by using the language of each people, they could proclaim the gospel to Persians or Scythians or Romans or Egyptians. But for anyone speaking in Corinth it was pointless to use the languages of the Scythians or Persians or Egyptians, since the Corinthians could not understand them." (CB, 230)
This is certainly a clear indication of the interpretation of "tongues" as "xenolalia." Theodoret portrays the ability to speak in foreign languages as real and applicable for the evangelization of the world.

"He who prophesies is greater than he who speaks in tongues, unless some one interprets, so that the church may be edified." 1 Cor 14:5b
"When there is no interpreter for what is uttered in tongues, prophecy is better because it is more beneficial." (CB, 231)