well, the reason, why en:gustaf looks bad with oaces lies at least in part in the ns_urlencode, which i did not want to cirumvent. wikipedia uses unencoded colons in links (see
http://de.wikipedia.org/wiki/Bild:Netz_ASEAG_um_1933.png). Anyhow, moving from en:gustaf to en/gustaf is a simple step, and makes the user think, there is a directory of english pages, which sounds like a good metaphore to me. this is certainly as well a very crawlerfriedly version. Using /en as a suffix sounds not so good to me, but this is rather a subjective argument. by any of these nameing rules we are reducing the range of valid names; e.g. "a/b/c" is so far a valid name of a page, Telling a user, "use $lang/name" sounds to me as a more intitive rule than "name must not end with /$lang" or similar. The overall wiki conventions are based on prefixes, so why to switch to suffixes. Certainly, using "?lang=en" is an option as well, but the basic idea of nice urls is to make these free of query parameters...
making the language optional is not something i would emphasize right now, since this can lead to serious misunderstandings, depending on how the default language is determined. If this is based on the user preferences, a link like pages/gustaf would lead to a page of the users prefered language. however, what if the pages in these languages are sparsely populated, etc. If two people look at a page wuth the same link and they see different pages, this can lead to some confusion. The automatic language completion sounds more suited for a well maintened set of pages (e.g. learning materials) than for a rather chaotically developed set of information chunks. So by default we should be precise on the links. btw, the language can be made optional in all variants.
Remember, we are talking about the links showing up in the listing of all pages, or from the category listings, or between wiki pages. Within a wiki page, a user will/should use [[gustaf]], or [[en:gustaf|Gustaf Neumann]] or whatever to refer to appropriate page. The links from wiki pages are generated at the time of rendering based on the language preferences. We are talking about beauty of these generated urls....
