Praying Privately in Tongues is Scriptural

I CORINTHIANS 14:28
28 But if there be no interpreter, let him keep silence
in the church; and let him speak to himself, and to God

In the above Scripture, Paul allowed a person the option of speaking in tongues privately to God.

I CORINTHIANS 14:14-17 [With my comments inserted in brackets]:
14 For if I pray in an unknown tongue, my spirit prayeth,
but my understanding is unfruitful.
[Praying in tongues exercises my spirit, but my mind doesn't benefit]

15 What is it then? I will pray with the spirit, and I will
pray with the understanding also: I will sing with the spirit,
and I will sing with the understanding also.
[I have two different ways of praying or singing available to me: praying or singing with my spirit (i.e., in tongues, as in verse 14) and praying with my understanding (i.e., in my known language).]
["I will" is mentioned four times, indicating that it was the Corinthians' own responsibility to choose the most appropriate way in which to pray or sing, in any given circumstance.]

16 Else when thou shalt bless with the spirit, how shall
he that occupieth the room of the unlearned say Amen
at thy giving of thanks, seeing he understandeth not what
thou sayest?
17 For thou verily givest thanks well, but the other is not
edified.
[If you give thanks with your spirit (i.e., in tongues) although your giving of thanks is none-the-less valid, your guest can't benefit because he doesn't understand what you're saying.]

18 I thank my God, I speak with tongues more than ye all:
19 Yet in the church I had rather speak five words with
my understanding, that by my voice I might teach others
also, than ten thousand words in an unknown tongue.

[The appropriate way to address a public gathering, is with your known language (or at least with an interpretation). But outside the public gathering you're welcome to speak in spirit - that is, in tongues without an interpretation - as often as you like!]