Yes, but being French, I think "nous nous sommes montrés les plus forts" could be translated as "we showed ourselves stronger", because you are stronger than the other people there... In this case both the comparative and the superlative can be used with the same meaning.
As long as you still have the article 'the', stronger would also be correct. 'We have proven ourselves the stronger' is the correct way to say this if you are only comparing two teams, for example. 'The strongest' is for comparing three or more. The Italian sentence doesn't specify a number, so both 'the stronger' and 'the strongest' should be accepted. :)