I want to use the google ajax api to get news in different languages
and display them with a tab control. The usual result does not know
the language, but I can use a context parameter. Unfortunately the
result then looks like this:

callbackFunction(
  contextValue,    // the context arg value
  responseObject,  // the collection of results and cursor
  responseStatus,  // 200 on success, non-200 on failure
  errorDetails)    // error string for non-200 response

note 4 parameters here. http://code.google.com/apis/ajaxsearch/documentation/reference.html#_...

The request uses onComplete : this.displayResult.bind(this) which
works fine without the context param, because the API answers with
only one param then.

I had a look into the code of mootools-1.2.4.1-more and replaced
'data' with 'arguments' here:

Request.JSONP.request_map['request_' + index] = function(data)
{ this.success(arguments, script); }.bind(this);

Now my function is called with an array of all the api arguments.

Is it safe to continue with this little diff or did I miss any side-
effects?

That looks reasonable; I think it would make a good change in the code; I'll
update the class for the next release.

To make it work properly, you'll need to update the success method:

 success: function(args, script){
if (script) script.destroy();
this.running = false;
 this.log('JSONP successfully retrieved: ', args);
this.fireEvent('complete', args).fireEvent('success', args).callChain();
 }

they key being that the arguments are applied as they are, and not wrapped
in an array (so fireEvent('complete', args) not fireEvent('complete',
[args]).

Now your complete event will be passed all the arguments:

myJSONP.addEvent('complete', function(contextValue, responseObject,
responseStatus, errorDetails){...});