undefined behaviour with default language and query param language set

[DanielHabenicht]

Describe the bug
If both the lang: 'en-gb' option (in settings.json) and the query param ?lang=de is set the displayed language is not really defined. Sometimes it displays english, sometimes its german.

To Reproduce
Steps to reproduce the behavior:

1. Set this setting in settings.json:

    "padOptions": {
      "lang": "en-gb"
    },

2. Start the server
3. Open an etherpad with the lang query parameter, e.g. https://localhost:9001/p/7j6qm9rldicn9ec1vpr4?lang=de
4. See the english or german etherpad.

Expected behavior
The default language set in settings.json should be overridden by the query param.

Server (please complete the following information):

• Etherpad version: 1d7a41f
• OS: Ubuntu 20.04
• Node.js version (node --version): v14.17.6
• npm version (npm --version): 6.14.15
• the server free of plugins

Desktop (please complete the following information):

• OS: Windows
• Browser edge
• Version 100.0.1185.39 (Official build) (64-bit)

Additional context
While debugging I found that this function is called twice in quick succession:
https://github.com/ether/etherpad-lite/blob/34d12022a076d967dcbfc0a3456687c55b4ea5a0/src/static/js/pad.js#L138-L141
But I could not find the part where it is called with the default language.
I would propose replacing the additional call with a variable which would replace the hardcoded 'en' string.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.