less $lang->load() part 0: JLanguage::load() accepts an array of paths

...to search for language files.

allows a single line of
```   $lang->load('plg_'.$folder.'_'.$element, array(JPATH_ADMINISTRATOR,JPATH_PLUGINS.'/'.$folder.'/'.$element), true, false, false);```
to perform the same as a bulks of
```   $lang->load('plg_'.$folder.'_'.$element, JPATH_ADMINISTRATOR, null, false, false)
|| $lang->load('plg_'.$folder.'_'.$element, JPATH_PLUGINS.'/'.$folder.'/'.$element, null, false, false)
|| $lang->load('plg_'.$folder.'_'.$element, JPATH_ADMINISTRATOR, $lang->getDefault(), false, false)
|| $lang->load('plg_'.$folder.'_'.$element, JPATH_PLUGINS.'/'.$folder.'/'.$element, $lang->getDefault(), false, false);```

I'll add subsequent pull requests where this change is used. 
Next part will handle "tpl_" files.