Multilanguage Contributing README (#25170)

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selfdrive/ui/translations/README.md

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+# Multilanguage
+
+![multilanguage_onroad](https://user-images.githubusercontent.com/25857203/178912800-2c798af8-78e3-498e-9e19-35906e0bafff.png)
+
+## Contributing
+
+Before getting started, make sure you have set up the openpilot Ubuntu development environment by reading the [tools README.md](/tools/README.md).
+
+### Adding a New Language
+
+openpilot provides a few tools to help contributors manage their translations and to ensure quality. To get started:
+
+1. Add your new language to [languages.json](/selfdrive/ui/translations/languages.json) with the appropriate [language code](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) and the localized language name (Simplified Chinese is `中文（繁體）`).
+2. Generate the translation file (`*.ts`):
+   ```shell
+   selfdrive/ui/update_translations.py
+   ```
+3. Edit the translation file, marking each translation as completed:
+   ```shell
+   linguist selfdrive/ui/translations/your_language_file.ts
+   ```
+4. Save your file and generate the compiled QM file used by the Qt UI:
+   ```shell
+   selfdrive/ui/update_translations.py --release
+   ```
+
+### Improving an Existing Language
+
+Follow the steps above, omitting steps 1. and 2. Any time you edit translations you'll want to make sure to compile them.
+
+### Testing
+
+openpilot has a unit test to make sure all translations are up to date with the text in openpilot and that all translations are completed.
+
+Run and fix any issues:
+
+```python
+selfdrive/ui/tests/test_translations.py
+```