I have to change the default language in/admin/globalsettings#language

Difficult to answer if we can't reproduce it.
I change the language in my account settings without any problem. And as said before, in 6.8.2.

I also realized that these strings seem to be not included in the po-file of the translation page.
Therefore I am a bit confused.
But this first string "Create a new survey from scratch ..." was implemented in update 601 of the database
 
Some time ago; the recent database update is update 623.

Unfortunately we can't compare with the cloud version as here we see the "new" GUI without this welcome page.
And - easy to understand - here a lot is still not translated.

So maybe it is due to this "transition".

Joffm