similar to what kierownik said, with my extentions i added to the hook "co_common" (seems to be the first one found)
if (file_exists(PUN_ROOT.'extensions/<extension>/lang_'.$pun_user['language'].'.php'))
require PUN_ROOT.'extensions/<extension>/lang_'.$pun_user['language'].'.php';
else
require PUN_ROOT.'extensions/<extension>/lang_English.php';
also in the install block i add this
if (!file_exists(PUN_ROOT.'extensions/<extension>/lang_'.$pun_user['language'].'.php'))
$notices[] = 'This Extension does not support your current language and will use the english one.';
replacing <extension> with your extension name of corse
With this scheme you don't have multiple folders with one file in them