As far as I am aware, it's the same files for all objects regardless of whether you have the German version (as I have, since there was no English version when I bought it!), or the translated version. This would make sense, as it would simply be creating work to rename the graphics and program files instead of just putting in a few translated graphic interface files and a manual in English to create the other version. In other words, any mods will work in any version as far as I know.
The textures are just like any other sim, and just as easily moddable, and in general, it's easy to get at stuff to change it, so yes, it is very 'moddable'.
I don't understand why people are so down on this sim, sure it has a few problems, but there's definitely fun to be had with it, and it's not as if there is another WW1 sub sim alternative.
When I am not quite so busy, I might have a crack at improving some of the textures, but as it is, since you cannot zoom the external camera, I don't think it's a major issue at the moment!
Chock