I am of the opinion that we should always reference sources if we're not writing directly from our own knowledge. If you have sources open in front of you, we should be open about disclosing that fact, and what sources those are.
Okay, I'll back up a little. I prefer to write on subjects where I'm something of an expert. That means I'm often writing from my own knowledge, acquired through the years from a number of sources. Then maybe I use a google search to look up a certain date. I come up with a mediocre website. When I list the references for the article I'd rather not list the website I looked up this one fact on. Instead I'll think "what would be a good English source for this?" and maybe list some book I read a year ago but don't have in front of me.
In happy cases, which is more often than not, I do have the book I want to reference right in front of me.
Inclusion of references includes foreign language sources, appropriately labeled, as you cannot assume that no user who understands Icelandic will ever read the articles.
Of course someone eventually might but that's not the point. The reference would be completely irrelevant information for the vast majority of readers. Worse, it would be misleading, implying that this Icelandic ornithology book contains something not found in numerous works in English. At best it belongs on the talk page or in the edit summary.
Citing non-English sources is, however, completely appropriate when corresponding English ones are lacking. There's nothing wrong with citing works in Hungarian on relatively obscure aspects of Hungarian history.
Full references should be listed at the bottom of every article (ideally, even below the category listings) where they have very limited ability to distract the reader.
I think a good bibliography can be an important part of the article and should be displayed reasonably prominently. The best bibliographies tell you a little bit about the books - look at [[User:Jallan]]'s bibliography for [[Norse mythology]].
A short overview on the major works on a subject is, when appropriate, one of the most useful things you can find in an introductory article.
Regards, Haukur