What I wanted to express is: this would be too big challenge to rename files... It seems to me easier to keep the paths "untouched" and link those posts with another way (eg. translationKey).

There is no other way to link translations together, you must use the same names for the files.

but I can't use foo.en.md and foo.fr.md and have slug bar for foo.fr.md (IIUC)

You can absolutely do that.

OK, I will add translationKey during "index" pages scraping (I already have lang known based on the original path location) and link articles together, and later rename filenames:

{
  "<uuid-based-on-the-first-title>":
    [ 
      '/orig_postA.html',        # first detected post
      '/orig_translationX.html', # detected translation
      '/orig_translationY.html'  # other translation
    ]
}

And later:

orig_postA.md = orig_postA.md ...and... orig_translationX.md -> orig_postA.fr.md

Unfortunately, the users can't find 'orig_translationX.md' but need to "grep" for the orig_translationX.html's title to find out new location for the old article.

Does it make sense? :)

Yes. Users looking for the translation specifically will need to grep, but users who want to find a post’s all translations will have it easier.