I'm not an expert in this matter, but believe in a miracle of human body.
I am not fully sure what you mean by "miracle of human body" but I can agree that us humans are capable of extraordinary feats that can be seen as 'magical' or 'miraculous'. Only because we haven't the ability to study or even detect the processes and mechanics of how these feats are accomplished. Without that data it may not be possible to fully accept certain occurrences as believable or based in fact or reality.
But then, there are cases when they start speaking different languages, changing voice, etc.
Speaking multiple languages is a common skill. Changing the tone of ones voice is not too difficult a task either. From a skeptical standpoint this is not concrete evidence of a supernatural event as it can be done by millions, possibly billions of people.
Under enormous pressure and stress people do amazing things when defending their offspring or saving themselves in danger.
Yes, the body stores many chemicals inside the brain and other organs which can be released in times of stress to assist us by boosting reaction time, speed, mobility, sharpening the senses, raising the threshold for pain and moderating blood flow. In the course of our daily lives we are actually using just a mere fraction of the full potential of our physical capacities.
And we only use something like 30% of our brains. What happens with the rest of it?
I'm just musing here but I get the impression that the brain is much like our muscular system in that we are not continually lifting heavy objects just like we are not continually solving complex math equations.
Also, even with our 'advanced' technologies we still know very little about all the functions of the brain and exactly how it works. There may even be processes happening with that other 70% of gray matter that we cannot yet detect.
We are exposed to ginormous amount of information that just gets lost in the brains and doesn't resurface in normal sircumstances.
You may have a point here. Who knows, maybe everything in our lives is completely stored within the brain. In this way every word foreign and domestic that has reached your ears may be stored and ready for repeating at any time. Perhaps it takes a trigger of sorts to unlock this wellspring of recorded speech to allow it to flow effortlessly from ones lips. It's an interesting thought.
You may never be aware what languages you have been exposed to through media and such, but it might just stay at the furtherst corner of your brain.