Navy in 1914.
commissioning West German Kobben class. Norway later resold some Kobbens to Denmark
lease a Nacken class to Denmark for four years. Sweden has done much better selling used subs to
Singapore and the new Collins to Australia.
Ten 212As have been built and 4 more are planned (2 each for the German and Italian navies). With Norway's initial indication it will buy 4 this means 18 x 212s. This provides increased economies of scale, resulting in lower costs/lower price.
The main reason for lack Swedish success in European sales seems to be NATO alliance (see Members) political solidarity, push to equipment standardisation and multi-decade strength of Germany's submarine industry:
towards European buyers seems stronger than the much smaller Swedish government and
The NATO reason especially impacts Poland as Poland realises it is too geographically close to the common enemy of NATO, which is Russia.
Another reason why NATO countries buy from NATO exporters is equipment interoperability concerns. This includes having complex weapons systems (eg. German and future Norwegian 212s) with similar performance characterists under the usual joint command scenarios.
See Kevin's comments recorded in this September 7, 2015 article which seem to indicate the Netherlands may tend to buy subs from Germany.