@r0lZ : thank you for you very complete explanation.
1st movie (before your answer) : I have checked on my TV (LG 55C6V) and...I haven't found how to active the subtiles...
I forgot to check the end of the movie as you said, nevermind it was just a 1st test.
2nd movie (after your answer) : I have followed all your recommendations and the subtitles ar directly visible. Unfortunately, I think my TV doesn't like Full SBS because what I see is a 3D movie with very large horizontal black bars
For your information, usually I always watch my movies on physical support (Blu-Ray 3D), but I am just preparing when I will watch my movies on a headset because my next TV won't have 3D capability