Short answer: a firmware update replaces system software. If Russian was supplied by a separate language package, the update may remove or deactivate that package.
Firmware and language are different layers
Firmware controls the device operating system, hardware support and core functions. A language package adds interface resources for a specific software environment. One can change without the other.
What to record before updating
- Exact model and hardware generation
- Current and target software versions
- How Russian was originally installed
- Backup of waypoints, routes and settings
What not to do
Do not immediately downgrade, factory-reset repeatedly or install a package for a similar model. Each action can remove useful evidence or introduce another fault.
Best next step
Send the model, old and new versions, and a photo of the language menu for a compatibility check.