How to instal english language pack
Just Google through the internet with the words "rx3715 language upgrade rom" and you will find several references to it.
At the end, you will find a couple of places where they have the lniks to access directly the HP/Compaq fttp sites, and download the ROM upgrade files in different language versions.
Also, in the web you will find a couple of sites that have the specific instructions and procedures to do it.
On purpose I am not giving you such sites and links because I have to warn you: such files and procedures work in 90% of the times. The catch is that such procedures forces the PDA to accept the new language rom, but in 10% of the times, such process is aborted with no apparent reason, and the PDA is converted in an useless PDA, but it is converted in a good looking paper weight or dead brick.
Bottom line, it's possible to change the language ROM (I did it a lot of times before: English, Spanish, Chinese, Italian and French), but it's risky, and it will damage permanently your PDA (such as I did in two of them).