After getting a file to update the hp iPAQ rx1950 from WM5.0 (Windows Mobile 5.0) to WM6.1 I figure out that this update was only possible to be done in an english iPAQ.
The file used to do the update is: Flasher_Hp_Ipaq_rx1950_ENG_WM6.1.rar (a russian hacked windows version translated to english).
The file that you need to modify to allow you to update an spanish iPAQ with a ROM for another language is hpRUU.exe. You only need to do a little change using an HEX editor. In the following picture you can see the little change that make the difference:
You can use any Hex Editor, my choice was KHexEdit. Also I do it from Linux but running a Virtual Machine (VirtualBox) with Windows XP.