IRC the French have at least one diesel sub, the SMX-Ocean Range 30,000km.
Wait so you honestly think they’re going 30,000 km on battery power?
Do you not get that submarine combustion engines are just like car combustion engines? You generate power through the engines and that power is stored on battery to power the electrical systems and serve as a backup, battery power is not going to power the whole entire sub and magically move the sub quietly through the water, that’s not how these things work, that’s how nuclear subs work.
My point is, you’re not going to be able to move your sub at all on battery power, at some point you will have to turn on those loud ass engines in order to move your sub, which will absolutely alert every modern sub to your location.