Ahoy, Dave!
First, we all know that Joomla 3.0 is just a short term supported version and that 2.5 is still a valid version for production sites (despite 3.x which is NOT reccomended for productions sites yet).
Second, I never said I can't create new articles. I said I can't access the original joomla standard form for articles, even when i click the link to use it (unless i uninstall seblod).
Third, I did just nothing of what you just wrote. All I did was installing Seblod and setting up some sites. I did not even touch any forms yet. I even uninstalled Seblod to see that articles then work again fine with joomla default form. But the problem reappeared after reinstalling Seblod WITHOUT making any changes on any forms. Besides this, while creating your account, I experienced that I have the same problem with the form to create new users.
Fourth, I'm not even interested in using any customized forms of Seblod. All I really need is the multisite feature.
Fifth, on my site english language is still installed on both frontend and backend - you should just have switched it ;-)
Last but least: Now I have an answer but it doesn't help the least for I haven't done any changes witht the forms. So the remaining question for me is: WTF shall I do now to fix it?
Thanks in advance and kind regards,
Me
P.S.: the article you created in my site does NOT use Joomla standard form, btw.