Force ajax requests to append the current language.

A bit of a hack, as:
* `doRequest` can be used for non-distributed server API calls (assumption: the parameter would simply be ignored)
* The `lang` parameter might already be set (but that's not the case at the moment)
* Other places might use `$.ajax` instead of this function (but we don't really want that; see #328)

This PR turns:
![image](https://f.cloud.github.com/assets/4072455/2284253/34f665be-9fc7-11e3-9efb-34b40ea6d742.png)

into:
![image](https://f.cloud.github.com/assets/4072455/2284255/43eb189e-9fc7-11e3-9867-4537835e8d29.png)