[core] fix slowdown when copying ISO files

* This slowdown was introduced in 897becd29015e384754a9fa7728cb4ad14a11cb2 as the use of
  SetThreadUILanguage apparently incurs a massive performance drawback.
* Closes #259
* Also fix a possible issue where the blocking operation message could appear twice
* Also improve the French translation
[loc] fix crash when two Rufus applications are running

* Closes #261
* Also remove the need for a separate get_loc_data_file call to
  initialize the en-US messages
* Also fix a possible issue with setLGP