For WW1/WW2 I identify German subs as Uboats, mainly because in all the books I have read about them they are referred as U99, U505 etc...
For WW1/WW2 I identify British Submarines as ... Submarines. This is because in all the books I have read they are referred to by name as for example the Submarine Sea Tiger etc (Good O'l John MIlls)
As for modern German boats... I have not really thought about it as I have not read much about them.
Cheers
Gary