File Systems – Why Can’t Files Be Inserted Without Additional Writes?

It sounds to me like you will simply be handing these files to another piece of software to actually read them in. If that's the case, use python or ruby or whatever easy to use high level language you have on hand because this program isn't actually IO intensive.

Now, if you are actually reading the contents of files in yourself to process on your own then I would say if the language you normally use doesn't have any interoperability for making OS level calls, don't use that language.

Ideally in this case you'd want a high level language with low level facilities.

C# for instance does this well in allowing high level handling of the simple stuff like processing the user input from command line and organizing the steps of your task or whatever else, but then allowing OS calls and direct memory management for highest possible performance (if necessary!). Java may do similarly? Not certain. Haskell is very high level as well as having facilities for direct memory manipulation, though Haskell has an extremely high barrier for learning if you don't already know it. C++ is probably the most commonly used language for this type of task for this exact purpose, it is a high level language and with C in it's roots it has complete low level facilities available.

That said, beware of premature optimization. Writing a program in a language that isn't your strongest language will likely under perform the one you could write in your native language as you wouldn't be aware of available optimizations or be using the language appropriately. Further the only way to know if the quickest most robust route of using your main language won't work is by giving it a go which would be far quicker than going all in on a language you don't know. So prototype something and see how it performs, if you don't think it does well enough then whip up a quick prototype in other languages and compare.

Prototyping as a part of your technical solution analysis is an ever important skill for all programmers, so take advantage of this task to practice it. In the scientific method we don't try to prove our hypothesis as that leads to bias, rather we try to disprove them. Your hypothesis is your normal language won't perform well enough for this task, so start by trying to disprove that.

Edit: Based on your edit, your program is absolutely not IO intensive, so write it in the language you are most comfortable with

Since you know which file is changed but not by which process this is hard but solvable. You can use psutil to handle this.

With psutil you can iterate over all running processes and ask them for their open files. Written as pseudo code:

for process in psutil open processes:
    for file in process all open files:
        check if this your file:
            yes => remember pid
pid is set:
    yes => wait for the process to finish
do your work with the file

This is an easy but not very performing way to handle your problem.