To my understanding it would seem kind of vain for someone to attempt to speak only in a language he himself understands. What would be the purpose of even doing it at all?
Ive reversed the order of your question as I think that this could make it a bit easier for a reply.
I should point out that no one will ever be able to understand what they are praying in tongues (1Co 13:2) as the Holy Spirit intercedes on our behalf to the Father in an Angelic/heavenly tongue (1Co 13:1). In 1 Co 14 Paul goes to some length to chastise the Corinthians who are praying/singing in tongues all at once during their congregational meetings; when we go to 1 Co 14:6-28 where Paul says that unintelligible speech, even if it is directed to the Father in prayer/praise, that this does not serve to edify anyone but the person who is praying to the Father in the Spirit so it must stop.
In spite of Pauls strict admonition that this practice must cease and that he has given good reasons why this is so, if you were to visit many Pentecostal congregations you will see many carelessly disregarding Scripture as to why, I wish I had a good reason to explain our carelessness in this matter, who knows, maybe its symptomatic of our needs based culture where we simply want to do what makes us feel good; but this is only a guess.
People of different languages understood on the day of Pentecost. Then why does Paul say that people speaking in tongues do so for their own edification and that you must "prophesy" in order for others to understand?
As the Day of Pentecost was a unique and unrepeatable event, we can understand why the tongues of fire and sounds of a rushing wind occurred; as this day was the inauguration of the Holy Spirit being given to the Believers, we can also understand why the Spirit chose to allow many of the Disciples to speak in known human languages. If the Spirit had decided to speak through them in an Angelic/heavenly tongue then this remarkable eschatological event would have been completely missed by the nearby Jews; the entire audience would have walked away deriding the Galileans as being drunk or probably even crazy (as per 1Co 14:23).
But what's puzzling is this. If "speaking in tongues" is where everyone hearing the message hears it in his own language, why does Paul say not to do it unless you have an interpreter?
The ability of either the unregenerate or even Believers to understand a word given in a tongue is rare and a very unusual event where I never expect to be in a situation where this will occur. Other than with the Day of Pentecost we have no examples of where tongues has ever been used to evangelise an unreached people group and Paul explains that whereas tongues is always directed to the Father in an Angelic/heavenly tongue, on the other hand prophecy is always spoken through the Spirit to an individual or group of people in their own language.
For that matter, it is worth pointing out that with the Day of Pentecost that the Holy Spirit did not speak to the Jews with an evangelistic style message but that they were words of praise about the Father which essentially were still being directed to him as well.
As Peter had to provide an evangelistic message where he could explain to the Jews what was happening, when we add to this the complete silence from the Scriptures with regard to the possible evangelistic use of tongues, then we can safely presume that we would only rarely, very rarely, ever encounter tongues being given in a known human tongue; but this does not mean that it cannot ever happen but only that it would be a very rare thing.
To summarise:
When we pray in the Spirit, the Spirit will always speak to the Father in an Angelic/heavenly tongue which no one will ever be able to understand; this applies to both our personal times of prayer and to its application within the congregational setting (3 tongues + 3 interpretations). When the Spirit speaks to an individual or a group this will always be through prophecy which is always given in the local language.
Does the Spirit ever reach out to an unreached people group through tongues; even though we have no Scriptural evidence for this, we it does happen we can at best say that it will be a rare (and problematic) occurrence.
You will often come across the formual
tongues + interpretation = prophecy, but this is one of those oddities that have developed within the Pentecostal movement but it can be discarded as the Scriptures do not provide any evidence of this - yes, even we Pentecostals have our odd moments and beliefs.