Plugin frontend helper to get the client language code

[JohnXLivingston]

Describe the problem to be solved

In the livechat plugin, i want to add some links to the online documentation.
This documentation is translated in the same languages as Peertube.
So, i want to add the current user language code in the url, to point to the correct documentation page.

It seems the languages is calculated by Angular, with a logic that returns the first found criteria:

• cookie clientLanguage
• browser language
• english

Describe the solution you would like

I don't want to reproduce the Angular logic in my code, and i'd like a helper function that returns the current language code.

I need this for frontend, but maybe it could also be useful for backend? I don't know if the backend knows the user's language?