Some clarification regarding those World War Two era submarines as sold by the Chinese to Bangladesh!
First there was Type XXI, then there was Whiskey, then there was Romeo, now there is Ming! Seventy years and going strong still!
From the Internet web site "Indian Strategic Studies".
"Two Ming III-class submarines will join the Bangladeshi fleet in 2016"
"These will be employed principally as training vessels; Bangladesh has not previously boasted a submarine fleet of its own. The apparent intent is to subsequently acquire more advanced diesel-electric submarines from either Russia or South Korea."
"The Ming class of Chinese submarines are an adaptation of the diesel/electric Romeo class submarine built in the Soviet Union, which are based on the German Type XXI submarine of World War II."
A Ming on the surface. Also called Project 035 by the Chinese.
"The Central Military Commission ordered the building of the Ming class submarines in 1967 as Project 035. The construction of the first began during October, 1969 at Wuhan Shipyard. The last boat was built in 2002"
See at this web site Chinese sailors in training for duty on a Ming. That appears to be a simulator and not an actual boat! Lots of advanced electronic systems as not found on a German Type XXI from the era of the Second World War.
DO NOT PRECLUDE THE POSSIBILITY OF THESE VESSELS HAVING AN OFFENSIVE CAPABILITY OF SOME SORT BEYOND MERELY A TRAINING MISSION.