hey marco,
for the first one COM_EASYSOCIAL_STREAM_CONTEXT_TITLE_FOLLOWERS I searched the whole easysocial package and this string doesn't exist at all - not in the english file and also not in the german file. also as the translations are at 100% this means that string is missing in the core files, not the translations.

for the second thing with "sidebar umschalten": to be correct you can't use your suggested "sidebar einblenden" because this sidebar text is shown on mobile all the time no matter if the sidebar is collapsed or not, therefore it would be wrong to say "einblenden" if it's already there ^^

but in general if you want to change some of the text you can easily do this for your site by using language overrides, more you can find here: http://stackideas.com/blog/how-to-override-joomla-language-strings

in fact that's always the best way to go, we all need different words depending on our sites, but the global language files have to be in a way that most sites can use it

Oh Alex

Yes, maybe its a missing string in core files yes, so the core files "also" have to be updated. But i'm telling you so you know it because i am using german package.
100% or not..

and for second thing i already have translated to another phrase by "overrides". But thank you for explaining me how to use language overrides

Nice weekend to you

ok, will let the devs know about this.
nice weekend for you as well