community/pdfgrep: prevent crash if LANG is not set explicitly

Locale support is really beyond my mental capabilities, however, the proposed patch here solves the issue of pdfgrep crashing on startup for me:

terminate called after throwing an instance of 'std::runtime_error'
  what():  locale::facet::_S_create_c_locale name not valid
Aborted

The patch description outlines some alternative solutions.