Sorry for interrupting .....
.... because this is a completely different topic:

I receive an error after applying language pack, which refers to "_MD_A_PREFS" and newbbhack.

error:
Notice [PHP]: Use of undefined constant _MD_A_PREFS - assumed '_MD_A_PREFS' in file modules/newbb/admin/index.php line 46

line 46,47:
<td><span class='fg2'><a href='<?php echo XOOPS_URL.'/modules/system/admin.php?fct=preferences&op=showmod&mod='.$xoopsModule->getVar('mid')."'>"._MD_A_PREFS;?></a></span></td>
<td><span class='fg2'><?php echo _MD_A_PREFS_DESC;?></span></td>


Somebody knows how to solve this?

It means that the constant _MD_A_PREFS is missing from modules/newbb/language/xxxx/admin.php, where xxxx is your current language.