
You will want to make your custom format generate the file names you want in the first place. If you want French titles, then you can select
French in the
language preferences. If you want French titles, but only if it's a French, or only if there's a French audio stream, or both, or something else, then the format can do that fully automatically. If you tell us your use case then we can probably make it work.
e.g. just use the primary title aka French title for French movies:
e.g. use French title if it's a French movie, otherwise use the default title:

That said, you can copy & paste the file path by selecting the item in
Original Files and hitting CTRL+C or by using
Double-Click <Original Files item> ➔ Rename before
Edit Name if you need the original file name in your clipboard before you start your edit.