this thread is going way too long, so I try to finish it up now ^^ let's split things:
1) first issue was that the comment and create blog stream were not changing the language - while checking files I noticed that the languagestrings weren't used but there were hardcoded english parts in the stream php files. you updated these files in the blog app and added 2 new language strings (btw: much better strings, that's the way all have to be) and now these 2 are working fine - RESOLVED!
2) now horst replied that it's still not working and he was testing this with the "featured blog post" stream in easysocial. so I checked the featured.title.php from the blog app to see if there are hardcoded strings as well but there aren't. also the languagestring used inside featured.title.php are in the plg_app_user_blog.ini so it was strange that it didn't work. by investigating further I found that the SAME language strings are used in the plg_app_user_article.ini file (APP_BLOG_STREAM_FEATURED_TITLE). in short: the system doesn't use the languagestrings from blog.ini but uses the one form article.ini - so whatever changes you make in the blog.ini for the featured stream have no effect because the system uses the string in article.ini - so you'd actually have to change the language in article.ini to make blog streams appear correct, I think nobody would know that
so if these aren't used at all I think they should be removed to avoid any confusions. also the article app isn't available on transifex therefore horst only had english stream at featured articles. I now made a german translation for him so the problem should be fixed for now at his site but others still may run into this problem
3) while you're right that most of these strings in the article app aren't used at all from my description above you see that it still causes problems by conflicting with the blog app. additionally there ARE used strings, especially the featured string. when you check the featured.title.php at apps/articles you'll find out that it uses the APP_BLOG string the same way as the featured.title.php at the blog app does.
I hope that makes lots of things clear and not generates new confusion