Add translations documentation

Translation.md

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+# Translation Guide
+
+Roster uses standard GNOME gettext infrastructure for internationalization (i18n).
+
+## Quick Start for Translators
+
+### 1. Generate the Translation Template
+
+From the project root, after setting up the build:
+
+```bash
+meson setup builddir
+ninja -C builddir roster-pot
+```
+
+This creates `po/roster.pot` with all translatable strings.
+
+### 2. Create Your Language Translation
+
+```bash
+cd po
+msginit -l LANG_CODE -o LANG_CODE.po -i roster.pot
+```
+
+Replace `LANG_CODE` with your language code (e.g., `cs_CZ` for Czech, `de` for German, `fr` for French).
+
+### 3. Translate the Strings
+
+Edit the `.po` file with a translation editor:
+- **GNOME Translation Editor** (gtranslator) - recommended
+- **Poedit**
+- **Lokalize** (KDE)
+- Any text editor
+
+### 4. Register Your Translation
+
+Add your language code to `po/LINGUAS` (keep alphabetically sorted):
+
+```
+cs_CZ
+de
+fr
+```
+
+### 5. Test Your Translation
+
+```bash
+ninja -C builddir
+ninja -C builddir install
+LANG=your_LANG_CODE.UTF-8 roster
+```
+
+## What Gets Translated
+
+The translation system covers:
+
+- Application UI (window.ui, dialogs)
+- Menu items and labels
+- Desktop file (application name, description, keywords)
+- AppStream metadata (for software centers)
+- GSettings schema (settings descriptions)
+- Python source strings (error messages, notifications)
+
+## Updating Translations
+
+When the application changes:
+
+```bash
+ninja -C builddir roster-update-po
+```
+
+This updates all `.po` files with new/changed strings.
+
+## Language Codes
+
+Use standard locale codes:
+- Two-letter: `de`, `fr`, `es`, `it`
+- With country: `pt_BR`, `zh_CN`, `en_GB`
+
+## Contributing
+
+Submit translations via:
+1. Pull request to the repository
+2. Issue on GitHub/GitLab with attached `.po` file
+3. Email to maintainer
+
+See [Contributing](Contributing.md) for more details.