In Germany they call all the submarines as Unterseeboot. If you use Unterseeboot, it means any submarines, rather than the military submarines used by Germany in both World Wars in particular. The game is clearly depicting those military submarines, they will chose the word U-Boat for it does refer to them specifically. On the other hand, compare to the Korean name "Geobukseon", we also have a "Confucius School" which isn't a Chinese name at all, and a Japanese name "Tera" that refer to all the Buddhist temples in general, so we can also safely assume that the devs don't have a standard on naming things. Which is understandable but still worth point out.