Hi there everybody,
We discussed the topic in our IRC meeting on Saturday and now I just created the page "Lists of European Monuments" on Commons (http://commons.wikimedia.org/wiki/Commons:Lists_of_European_Monuments). It's supposed to serve as a starting point when researching monuments, especially if you're not familiar with a country's subdivisions and/or language.
We won't create actual lists of monuments on Commons but rather link to the existing ones on the local Wikipedias. We could also think about linking to the corresponding categories as well, maybe even having a bot that runs statistics on the monuments categories and puts the results in a table/template (e.g. a column for "Number of unidentified monuments"). Endless possibilities...
But right now the most important thing is to get the lists up there and to put them in a logical structure. For Portugal this was quite easy, for Germany it's going to be tough...
For all the other countries, someone has to volunteer to create the respective page and to translate the information on the main page and eventually all the other pages. The main language should be english, the second one listed the language of the country the page is for and then alphabetically (or whatever ;) )
I hope that the concept has become clear, if there are any questions, please ask, if there are any great ideas how we could improve this list of lists and make it even more usfeul: go ahead and try!