I mean my grand uncles fought in the first world war(at least 1 in the western front around 1914-1915) and according to the snipets from my oldest aunt they spoke decent English, especially after they joined the army.

Furthermore the whole story is pushing a heroic narrative in a completely unlikely manner, no one would be tasked with such a tall order, especially not two random privates. The events along the way would never happen, especially not in one day. But for the sake of representing the first world war it's horrors and it's heros they shoved these things in. I see this part as a logical continuation of that. That wanted to show the the often forgotten heros of the war and this was one way of doing that.

Personally as someone who's grand uncle died in the war and others went to it I love this representation even if it may be shown in a somewhat unlikely manner.