Added "zxx" (no linguistic content) in available video languages

[Poslovitch]

Description

It adds zxx as part of the available "language codes" in the backend. zxx is the ISO-639 code for the absence of linguistic content, according to Wikipedia (https://en.wikipedia.org/wiki/ISO_639):

No linguistic information at all (added 2006-01-11). The content (e.g. graphics, photos or audio/video records not including text in a human language, or technical metadata and most programming source code) is usable as is with any language and should not be translated (except for its description possibly associated in separate contents, or for non-essential fragments of the content).

As this "language" also appeared in the group "All languages" in the forms, I had to create a dedicated group for it. Feel free to suggest better names 😉.

Related issues

Closes #1489

Has this been tested?

• 👍 yes, I added tests to the test suite
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this PR does not update server code
• 🙋 no, because I need help

Screenshots

In the video-edit form:

When watching the video :

[Chocobozzz]

Thanks!

[Poslovitch]

Well, thanks for merging ! ❤️

[ghost]

This is a harmful feature imo. The selected language should or could also reflect the language of the title of the video. Using "no linguistic content" would only make it even harder to filter out content in languages you do not understand.