chore: improve Russian language support (#926)

## Summary

This PR includes vocabulary and grammar fixes for Russian translation,
originally made as review comments
[here](https://github.com/crosspoint-reader/crosspoint-reader/pull/728).

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### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

docs/i18n.md

@@ -2,8 +2,16 @@
 
 This guide explains the multi-language support system in CrossPoint Reader.
 
-## Supported Languages (Updating)
+## Supported Languages
 
+- English
+- French
+- German
+- Portuguese
+- Spanish
+- Swedish
+- Czech
+- Russian
 
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