A.T. Robertson Commentary 1 Corinthians 14:28

"but if there be no interpreter, let him keep silence in the church; and let him speak to himself, and to God." — 1 Corinthians 14:28 (ASV)

But if there be no interpreter (εαν δε μη η διερμηνευτης). Third class condition. Earliest known instance and possibly made by Paul from verb in verse 27. Reappears in Byzantine grammarians.

Keep silence in church (σιγατω εν εκκλησια). Linear action (present active imperative). He is not even to speak in a tongue once. He can indulge his private ecstasy with God.