Unknown tongues just means tongue unknown to the speaker, not unknown to God who knows all tongues.
Also in Revelation, people of all tribes and TONGUES will be gathered. Revelation 7:9
Tongues is just another name for langauge.
If you have been given the gift it is for you to use to pray to God, but obviously when you are first given it there maybe other people around you who can interpret (as on the day of pentecost) but it may even happen that you are given the gift when theres nobody else around except God to hear
I would suggest you keep praying in your tongue in your closet alone with God. I think it has become a bit fashionable for christians to pray openly in public to seem pious, or in large groups.
Yes its important to learn and teach prayer, but often prayer then becomes a thing to show off in front of others. Long prayers can be the worst lol
Note on the book of acts when disciples went round they didnt all go and pray loudly to show off they were praying ..they prophesied, laid hands on people and healed them