Support French localization in the Gradio WebUI

The Gradio WebUI currently forces Chinese and hardcodes most labels, help text and status messages, even when a different PYVIDEOTRANS_LANG is configured. This makes the interface difficult to use for French-speaking users.

Please allow the WebUI to honor the selected language and provide a French catalog covering the main page, provider settings, advanced options and WebUI status messages, while retaining Chinese as the default and preserving backend option values.