I think I can help here, mate. I managed to get a Russian ROM working on my older iPAQ once (different model, but same OS). You might want to try searching for some online resources or Russian forums for the specific RX3715 version.
I don't think there's an English thread for this, but I can try to help. Have you tried checking the official HP forums or a community-driven wiki for your device's language settings? There may be a workaround to change it to Russian.