I have several collections in x265 in french on one seedbox and never grabbed yet so I wonder if I can watch them on my PC and if yes, then which program will play the files..

thanks in advance

I guess VLC will do the job.

Mpc-hc

@Rhialto saw that you wrote about it in the chat - i personally would recommend you the MPC-HC that comes with the K-Lite Mega Codec Pack.

i also just made a new guide for best Video Quality rendering - its really easy i think.

you can find the guide here: install / setup K-Lite Codec Pack with MPC-HC Player and madVR for best Video Quality

and of course you can play x265 files - btw for 4k x265 might be usefull and as it normally is used with 10bit gives more extra points to it, but for anything betweeen SD and 1080p i would alwys go for a x264 rip - its simply the best for now - and it could take many years till that changes one day, if that ever happens, x264 isn't dead and still in development.