I just got my old k750 repaired (for 500 bucks!) and after running the update service from the official SE site, my phone's stopped working.
Upon switching the phone on, I'm told there's a configuration error. Please contact your network operator or service centre.
I've read around that the phone needs to be finalized or something and you need to pay for it. What the hell, why does SE offer a service that will mess your phone up only for you to dish out more cash to get it sorted?
I never thought someone would ask this on My Broadband Forums, finally I can talk about something I actually know of!
First of all, RELAX! You don't have to pay for anything. Let me explain. Sony Ericsson firmware (the stuff you updated) consists of 3 things. The MAIN, FS (file system) and Custom files.
> Main: This is like GUI (graphic user interface) of your phone
>FS: this is the file system, including preloaded stuff like default wallpapers, and also drivers for the phone's components.
>Custom (where your problem lies): This is the language pack and service provider settings, that you are missing.
OK so what you need to do is flash the phone from the start.
Here is a VERY well written tutorial, step by step

:
http://forums.se-nse.net/index.php?showtopic=16148
You are going to have to register at SE-NSE forums to download the needed files though. But it is a great forum for everything you need to know about Sony Ericsson phones (I'm a proud member for very long now).
You can get the firmware files from here:
Here are the one's I recommend (have flashed k750i before)
> Main:
http://www.iprotebe.cz/download-K750_R1DB001_MAIN_1_EU_1_CL_RED36.rar-file736.html
> FS :
http://www.iprotebe.cz/download-K750_R1DB001_FS_EUROPE_1_RED36.rar-file731.html
Custpack :
http://www.iprotebe.cz/download-K750_CDA102337_10_Generic_Germany__South_Africa.zip-file4663.html
Sorry, you are going to have to register at IproTebe too, it's also free
OK so I would recommend that you rather post the exact topic you posted here on SE-NSE just to make sure what firmware to use and if you have any other questions. The people there will be able to help you flashing your phone, BTW if you thought the members on MB forums are sarcastic, you haven't seen anything yet! (keep that in mind when asking questions on SE-NSE).
I don't take any responsibility if you mess up your phone in any way!