I didn't see the 'twist' coming that the silent soldier was a French soldier trying to escape.
Twist!
Did you not notice that he was swapping uniforms and burying the evidence when Tommy first spotted him? There was no doubt in my mind what was happening at that moment. Tommy knew and was covering for him all the way, hence the "tell him" moment. The reason they didn't really speak is because they didn't really have a common language. He stayed up top on the ship so he didn't have to speak to anyone in the hull.
If you see it again I think you will wonder how you missed it! It was all a sub plot of excitement wondering when he would get rumbled!