The only meaning that can be given to the words you quote, 1 Corinthians 14: 18-19 (without any further information being available) is that Paul is saying that he preached the gospel to many people in different countries in their own language.
But, in the church, he speaks intelligibly, to those present.
Else, he would not be understood.
Anyone who, in the company of others in the assembly, speaks in a language which nobody present can understand, must only be talking to God, for God alone will know what the person is uttering, 1 Corinthians 14:2. Indeed, the person may well speak to God 'in the spirit', but what they speak will be 'mysteries' for 'no man understands him'.
To read any more into the words is to add meaning to them that Paul is not actually expressing.
As Paul makes clear, anyone speaking in a language that nobody present can understand, must first ensure that they provide an interpreter to communicate the meaning. Otherwise, they are not permitted to speak in the assembly, 1 Corinthians 14:27-28. They will have to remain silent.
Even when Paul is on his own, he tells us that he will pray with his spirit and with his understanding also, or he will sing with his spirit and with his understanding also, 1 Corinthians 14:15.
By using the construction δε και (but also) Paul is indicating a simultaneity to his activity, his praying/singing being in immediate conjunction with the activity of his mind.
"καί ... δέ, but ... also, yea and, moreover also" Thayer's Greek Lexicon
So even when alone, in his own devotions to God privately, Paul was not prepared to, himself, utter a language with his mouth which his mind did not understand.
So, to answer the question,
What did the apostle Paul use the gift of tongues for ? [OP]
. . . the evidence appears to be that Paul used language (as did the eleven on the day of Pentecost, Acts 2:8) to communicate the gospel to his fellow men in their own tongue.
It is very obvious where Paul's emphasis lies for, before even addressing the subject of language in detail, he first makes clear in Chapter 13 of his first epistle to the church of God at Corinth that [KJV] :
Though I speak with the tongues of men and of angels and have not charity, I am become as sounding brass or a tinkling cymbal ...
... whether there be tongues, they shall cease ; knowledge - it shall vanish away.
... Now abideth faith, hope and charity.
... But the greatest of these is charity.