text: Title: Add frontend language default parameter 
  Body: Hello there,
  first of all: really great work! 
  
  I would love to see a parameter/config/function to set the default language for the frontend. So that you do not always have to set them by hand.
  
  Here I would like to take over the implementation. But first I would like to clarify a few questions with you:
  
  - does this feature make sense at all?
  - Implementation via cookie in the frontend so it remembers for each user?
   OR
  - Add two cli arguments and integrate them into the frontend via template variables?
   OR
  - cli arguments and fetch in frontend via api. e.g. GET /languages/defaults -> { "source": "en", "target": "es"}
   OR
  - cli arguments and fetch in frontend via api. e.g. GET /frontend/settings -> { "source": "en", "target": "es", "darkmode": true}
  -> here already in view for  https://github.com/EsmailELBoBDev2/LibreTranslate/commit/68ce70cb78a43b94100ab3eca5ddc7fda9d49d8b
  
  I prefer one of the last two options. But I am not sure if this is a bit overloaded, because the main feature of this project is the translation api and the frontend is more of a showcase? (So this feature should rather be included in the project which uses this API...)?