I've never used this before, but here's what I did (1.9.1)

1/ changed $po_extraction = 1; ( conf/_locales.php)
2/ went to admin > settings > regional > click show translation info
3/ clicked extract next to german

I then got this on the screen :-

Sources and number of strings:

* _header.php = 46
* a_noskin.php = 16
* conf/ = 100
* contact.php = 4
* cron/ = 44
* default.php = 25
* htsrv/ = 81
* inc/ = 1724
* install/ = 134
* multiblogs.php = 4
* plugins/ = 187
* skins/ = 126
* skins_adm/ = 15
* summary.php = 3

and I found _global.php in /locales/de-DE/

Does none of this happen for you?

¥

I guess he did not click "show translation info".

btw: you probably changed $allow_po_extraction, not $po_extraction, Yabba.