feat: Auto-detect the user's language or set a default language.

[PasserDreamer]

Is your feature request related to a problem? Please describe.
When accessing the WebUI for the first time or from another device, the language is set to English, and it remains unchanged even after login, even if the language setting was previously altered during an earlier session.

Describe the solution you'd like
Allow the WebUI to detect the user's browser language and automatically change the locale.
Add an environment variable WEBUI_LANGUAGE to set a default language.

[justinh-rahb]

Edit backend/config.json or use this command for Docker install:

docker exec -it open-webui sed -i 's/"en-US"/"<locale>"/g' /app/backend/data/config.json