Well... technically... that switch is VERY dangerous, but if you FLIP it it reboots the system, and if you HOLD it for 2 seconds it defaults the system back to factory (German) defaults. Unfortunately if you don't flip it fast enough you find that out the hard way.
And if you do that, the "oh shit" procedure (for English) is as follows:
Using your switchroom phone which should have been installed for this purpose:
Dial * 4 8 1 2 this sets the PHONE to English
Dial 3 1 9 9 4 this is the default user name
Dial 3 1 9 9 4 this is the default password
Dial 3 1 9 9 4 this selects a NEW password (leave it the same)
Dial 3 1 9 9 4 this confirms your new password choice.
Now Dial 2 9 to get to the system menu
Scroll with >> to the Init menu
Choose your country (US)
Save your changes.
Now fire up the laptop or administration PC and go into Assitant E - it is best to be hard cabled to the system at this point because your dial plan is still trashed from deing defaulted so you may not be able to dial in to the built-in modem.
Load your most recent save of your customer database (may be LASTLOAD)... You are making regular backups every time you make changes, right??
Transfer the Customer Database from PC to SYSTEM, MAKING SURE the "overwrite" option is checked. Normally you shouldn't use the overwrite option when sending the database, but in this case you want to completely overwrite the memory of the system, including all phone settings that were active at the time the database was saved.
Once that is complete, FLIP (read gently slap and get your finger off it) the reset switch to get everything back to stable and you should be good to go.
There is also one other method you can try that may be faster to reset the system to English but it doesn't always work and may be for the really older systems.
That is, from the switchroom phone, to dial:
* 9 5 3 1 9 9 4 7 8 1 1 0 3 7 0
written easier to read it is: *95 31994 78110370
Hope this helps someone out someday who gets themselves in a mess. I've had it happen to my backup tech once while I was on vacation for a week, and I have had to do it myself a couple of times when performing software upgrades to the system.