The night train Strasbourg 23:42 - Salzburg Hbf 7:26 is fully booked on 23 December.

For Prague to Paris, have a look a a travel planner, e.g. on bahn.com, to see which trains you want to use.

As rvdborgt said the night train is indeed full.

This means that you'll have to spend a night en-route in Germany. If you have accommodation booked in Salzburg could you travel earlier in the day ?

It could be possible to leave at 19:42 and arrive at 3am but that is not recommended at all.

For Prague - Paris have a look at your other post.

EDIT : Are you alone or multiple people ?

Unfortunately I can't travel earlier in the day 

But what do you think if I booked the train from strasbourg to Munich arrive 23:26 with seat reservation 

Then from Munich to salzburg 23:45 or 00:57 departure time 

Yes you could do that but of course you'll arrive quite late. Check if your hotel offers late check-in.

- TGV Strasbourg - München Hbf 19:42 - 23:26 17€ reservation (1st/2nd class)

- RB München Hbf - Salzburg Hbf 23:51 - 01:46 or 00:56 - 02:47 

I do not get the same departure times from München Hbf (main station) though.