Body Class, Drupal 7, change language name programmatically
sites/all/modules/i18n/i18n.module

Line:267:  $variables['classes_array'][] = 'i18n-' . $language-&gt;language;


You can create your own small module mysmallmodule.module and put this in it:

/**
 * Implements hook_preprocess_html().
 */
function mysmallmodule_preprocess_html(&amp;$variables) {
  global $language;
  $variables['classes_array'][] = 'lang-' . $language-&gt;language;
}


Which wont get rid of the i18n class but will just add your own version :)

EDIT: Step by step to create a small custom module.


Create a new folder in the sites/all/modules folder called custom, in here you can keep any custom modules you may need.
Create another folder in the custom folder and call it something suitable like customclasses.
In the customclasses folder create two files, customclasses.module and customclasses.info.
In the customclasses.module file paste the code above but call it "function customclasses_preprocess_html" instead of "mysmallmodule".
In the customclasses.info file paste

name = Custom Classes

package = Custom Stuff

description = Custom class to get "lang-" in the body class.

core = 7.x
Finally, go to the modules page and enable your first custom module.