feat: add internationalization (i18n) support

Add multi-language support using rust-i18n with 6 locales:
- English (en) - source of truth
- Spanish (es)
- German (de)
- French (fr)
- Chinese (zh)
- Russian (ru)

Features:
- Automatic system locale detection via sys-locale
- Manual override via --lang flag or RUSTNET_LANG env var
- Translated CLI help text, UI labels, error messages
- CI check to ensure all translation keys are consistent

The locale is detected before CLI parsing so that --help output
is displayed in the user's language.

Author: domcyrus

.github/workflows/rust.yml

@@ -9,6 +9,8 @@ on:
       - 'Cargo.lock'
       - 'assets/services'
       - 'build.rs'
+      - 'assets/locales/**'
+      - 'scripts/check-locales.py'
       - '.github/workflows/rust.yml'
   pull_request:
     branches: [ "main" ]
@@ -18,6 +20,8 @@ on:
       - 'Cargo.lock'
       - 'assets/services'
       - 'build.rs'
+      - 'assets/locales/**'
+      - 'scripts/check-locales.py'
       - '.github/workflows/rust.yml'
   workflow_dispatch:
 
@@ -34,7 +38,13 @@ jobs:
     - uses: actions/checkout@v6
     - name: Install dependencies
       run: sudo apt-get update && sudo apt-get install -y libpcap-dev libelf-dev zlib1g-dev clang llvm pkg-config
+    - name: Check translation keys
+      run: python3 scripts/check-locales.py
     - name: Build
       run: cargo build --verbose
     - name: Run tests
       run: cargo test --verbose
+    - name: Security audit
+      run: |
+        cargo install cargo-audit
+        cargo audit