Note 13 at 1Co 14:28: If no interpreter is present within the church (see note 12 at 1Co 14:27), then those with the gift of tongues are to keep silent. They are encouraged to speak quietly to themselves and to worship God. Although their gift does not benefit the church without an interpretation, there is much benefit to the private practice of tongues, or as it's stated here, within the church "let him speak to himself, and to God" (see also 1Co 14:4, 14, and 16-17). Softly praying and praising God in tongues in the assembly, as private prayer, is certainly in order.

However, the practice common in many Spirit-filled churches, of everyone praying in tongues loudly at the same time, is not consistent with Paul's instructions here. The people should always be able to understand the person leading the service unless he or she is delivering a message in tongues, which should then be interpreted. An individual should not be distracted from the service through another praying in tongues unless it is interpreted.