You know, I never knew that that sidebar was a link to the same entry in different languages - thanks! Still, it makes me wonder if there is still a way to open up more content in other languages, so that those who contribute more in-depth can somehow have that content be shared on other language pages more transparently. But, I never studied library science and I'm sure finer minds than mine have considered this problem.
The tricky thing is that any text content has to have translation. You might be able to get away with not translating maps, since place names tend to be more stable (or at least generally pretty easy to work out) across different languages. For example, "Pologne-Lituanie" is going to be within the capability of most English speakers to work out, even if they've never heard of "Poland-Lithuania".
It is possible to link images and other things via Wikimedia, and my understanding is that Wikipedia does push for people to do this.