Change language on a Pokemon rom hack ?

That really depends what was done for the hack and what version it was aimed for, and how the original game works. From memory then pokemon tends to be one language per version of the ROM when it came to the DS (though even the older stuff was much the same way) and not always a quick and dirty translation when going between Japan and the US and then the US and Europe (or maybe Japan and Europe of Nintendo Germany was handling the translations -- don't know what went for pokemon).
If the hack changes the story and text in the game (as opposed to just some stats) then it gets harder. Looking at https://gbatemp.net/threads/pokemon-sacred-gold-storm-silver.327567/ then it claims a whole bunch of new events which probably means changed text.

Short version. It would likely be a more involved hack than something I would be able to say "unpack this, unpack this, copy these, repack ROM" for. If you wanted to try overwriting font, text files and whatever else from the equivalents the official French version it is possibly worth a try but I would not expect much.

 

Anything that pulls apart a ROM into its constituent files (ndstool, dsbuff, dslazy, crystaltile2, tinke, NitroExplorer 3, https://gbatemp.net/threads/release-ndsfactory-unpack-and-repack-nds-roms.548053/ .... the list goes on for a long time, probably avoid those that only extract and import individual files of the same size (so ndshv and ndsts).

Once unpacked you just use normal file copy and paste, and then you get to repack it just the same. I don't know what narc files are responsible for the language (or indeed what subset of them -- for this you might only be able to replace a fraction of them without reworking things) offhand but someone would have written them down, and you can always experiment.