How to change default language for configuration entities?
I changed the configuration directly in the MySQL table "config" (In my case table names has a prefix ("something_config") so change the example below if necessary).

The configuration values are in the column "data". 

First I cast the content to a string. 

Then I replace the definition of the wrong langcode. In my case I replace 's:8:"langcode";s:2:"de"' with 's:8:"langcode";s:2:"en"'.

In the end I cast the result of this replace operation back to binary and save it in "data". 

These three steps can be managed in one single SQL statement:

UPDATE `config` SET data = cast(
 replace(
    cast( data AS char ) , 's:8:"langcode";s:2:"de"', 's:8:"langcode";s:2:"en"'
 )
 AS BINARY ) 


Then clear the cache and you are done.