Here are a few notes...
(1) The non-English language title guidelines have existed since March 2010 (social.technet.microsoft.com/.../364.wiki-non-english-language-title-guidelines.aspx), and they didn't extend to the English language... Because the Wiki is for the English language.
There are many reasons to have the Wiki title codes for non-English languages...
(2) First, the URL output make it so that some non-alphabet languages "stole" the titles from English (and other) languages. For example, an article with "Hyper-V" and Japanese characters took the "Hyper-V" title so that we couldn't make an English article with the "Hyper-V" title.
(3) Second, people who don't speak a specific language didn't even know what language they were looking at. Since it was all in one Wiki world (sharing tags and search results), we wanted to make it a little more obvious what language we were looking at.
(4) The non-English codes in the title make it easy to search for that language (and the tags give you an instant list). So the title language codes were used to search for specific languages. You can exclude languages from your search, include multiple languages, and include a specific language(s) along with specific technologies.
(5) I think the "system" based approach you refer to might be the concept of Wiki instances that are divided by language. We've definitely been looking at this solution since early 2010.
(6) I can think of one good reason to include the "en-US" tag: if you're sick of seeing non-English articles in your tag list, then this is a way to exclude them (assuming every English article has the en-US tag).
(7) We made the "Multi Language Wiki Articles" tag for any articles that have been translated into a second language.
(8) I love your multiple-tag solution! I suggest you create an article that's focused on the message of "how to find SharePoint 2010 articles in different languages". It's almost like a sub-section to this article (so you can just copy and paste it), but really it's a different topic and worthy of its own article.
This is very good article. Please also see my today's (06/052013) suggestion about adding ability to specify languages and applying automatic filter for the content based on the languages. When I am browsing latest articles I would love to only see articles in the languages I am familiar with, not every article.