Hi, I created a questionnaire (Germany´- German). And I need the same questionnaire (Swiss-German). I tried to copy the questionnaire, but I can not change the base language. Is there any way I can copy the questionnaire and change the base language? Thank you. 

Hi, as far as I see, there is no swiss german included in LimeSurvey.
So, if you copy the questionnaire you may stay to german and change some wording.

Or did you create a new translation file "de-ch"?
Then it shouldn't be necessary to copy.

So I have no idea what you are planning to do and why.

But to answer your question.
Yes it is possible.
In the lss export of your survey exchange ALL occurences of "de" to the other language code.

Example:
<language><![CDATA[de]]></language>    ->   <language><![CDATA[nl]]></language>

Joffm

Or alternative way : export Tab separated survey and replace language in language column.