The splicing of the audio and video between language adaptations of the show is tricky, partly because the credits are different lengths in each localisation.
If you were to mux them together, you would find it tricky to sync the audio with the picture.
Another thing is that the differences between PAL and NTSC framerates make the episodes slightly different lengths and could throw the synchronisation off.
So, it's best to mux between two PAL or two NTSC sources.
The French DVDs of Star Fleet are produced from a 35mm master, unlike the UK DVDs, which are taken from 16mm.
Also, the French version has undergone a fantastic remaster and the colour and contrast are great, far better than the Japanese DVDs even.
So, if you were to attempt it, there seems to be a lot of software available for the purpose although anything decent will probably cost you something.
BladeZ is the person to talk to about this. He has been working on a remastering project. I know 80% of the above through him.
He runs the Star Fleet Wiki, which is great btw; http://star-fleet.wikia.com/
You should PM him on here and see if you can't help eachother out.
The UK DVDs are so atrocious in terms of VA quality that the audio isn't even stereo. I guess that's why the extras had to be so good.
There is a US release that came out just last month. Its picture quality isn't supposed to be fabulous but I wonder if it at least has stereo audio.
Here's a link that might be of interest:
http://originaltrilogy.com/topic/Star-F ... 833/page/2