Try something like this:
IF NOT EXIST (SELECT listid FROM list WHERE @languagecode='en' AND ListName_en =@listname AND ListType=@ListType AND AccessType=@AccessType AND (CreatedBy = @EmplpyeeID OR @CreatedBy = '0' OR @AccessType = 'Global'))
BEGIN
--some other statement
END
You do not need to use a case statement in this situation. You have multiple values you are checking against and a case statement will give you one result for each case statement so is not appropriate for your query.
[UPDATE]
Try this, it will get some values you can check before the query runs.
DECLARE @listNameChoice varchar(25)
SET @listNameChoice = (Case WHEN @languagecode = 'en' THEN 'ListName_en' ELSE 'ListName_DE' END)
IF NOT EXIST (SELECT listid FROM list WHERE @listNameChoice = @listname AND ListType=@ListType AND AccessType=@AccessType AND (CreatedBy = @EmplpyeeID OR @CreatedBy = '0' OR @AccessType = 'Global'))
BEGIN
--some other statement
END