[Typo3] Using locallang labels from sr_feuserregister in another frontend plugin

["Christoph Köpernick @ atkon"]

Dear list!

I´m working on an fe plugin, that shows user information. e.g. the first 
name of the logged in fe_user. I want to use the LL labels e.g. 
"first_name" from sr_feuser_register. I found that you can use 
pi_getLL() to get language labels that are defined in the locallang.php 
of the actual extension. But there seems to be no such function to 
import the locallang file from another extension.

I tried
$LANG->includeLLFile(t3lib_div::getFileAbsFileName('EXT:sr_feuser_register/p1/locallang.php')); 

but the $LANG object isnt available.

Can you please make some suggestions how I can use locallang labels from 
another extension in my new fe plugin?

thanks, chris