A question that must be brooched is "What is speaking in tongues?" Quite clearly those demonstrated in Acts 2 were human languages. Is the claim that people who speak today speak other languages? In my experiences in a Pentecostal church before I became a member of Christ's church, was that these were not human languages, but just a selesction of sounds made by well-intentioned men and women.
Refrus
Indeed, what is speaking in tongues?
"And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;" Mark 6:17
A new 'tongue' may be another language other than your own native one, but it may also be replacement (spiritually) of that most unruly member spoken of by James.
Do all speak in tongues? If a person is save he does speak a new language, the language of Christ and he also understands it (whether it is a language other than English may or may not be the case):
"And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables:That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them." Mark 4:11-12
The parables were the 'tongues' that Jesus spoke. Those that were to understand the mysteries of God were able to understand them. Those, however, without 'ears to hear and eyes to see' could not understand them.
As to the tongues of today some of which are actually others languages (as English is a language): At the start they are not languages at all but efforts which are appreciated and therefore understood by our heavenly Father. This is akin to a natural baby who babbles and makes sounds with no real meaning at first except to his parents. Then in time the sounds begin to form imitations of words spoken by his parents. Later the words become phrases and then complete sentences.
"For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little:For with stammering lips and another tongue will he speak to this people." Isa 28:10-11
At the start God speaks to us simply teaching us by speaking in effect a spiritual baby talk to which we will reply in kind. As we grow in Him, He then begins to speak to us in a more sophisticated manner which is 'another tongue' and we will be taught by Him to reply in kind.
"I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able." I Cor 3:2
Many people who have been drawn to Christ become stunted in their growth because they have become lukewarm in their love of and pursuit of Truth.
"For when for the time ye ought to be teachers, ye have need that one teach you again which be the first principles of the oracles of God; and are become such as have need of milk, and not of strong meat.For every one that useth milk is unskilful in the word of righteousness: for he is a babe.But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil." Heb 5:12-14
In the tongues of languages (similar to English) this growth or lack of growth is displayed by the advancement into a real language or the continuance of baby talk. God does not want us to remain babies all of our spiritual lives.
Whether we simply speak in a form of parables in English (or what native language we speak) or we speak in another 'unknown' language such as Chinese, it is another tongue 'from God'.
Why do we all not always understand each other if we are all Christians? Because all of are not on the same page and probably none of are completely fluent in the 'new language', but God, the Teacher understands everything that we say.
