Add a way to reset language setting and revert to automatic detection

[JulianKniephoff]

@lkiesow wrote in #1069 (comment):

“Appearence” has a mode “Auto” to select the system design. Language should also have an auto-option to select the preferred system language.

with a 👍 from @oas777.

[wdhowell84]

I started on a solution for this before running across how OpenCast already does this. If you navigate to OpenCast for the first time and your browser has a supported language as its preferred language, i18next picks up that language and defaults to it. If one were to go forward with this issue, the auto could possibly clear out the local storage and force i18next to rediscover the preferred language in cases where you visited OpenCast without your preferred language defined in the browser or had changed after the fact.

[LukasKalbertodt]

Yeah I think that's exactly the plan: clear the local storage for i18next so that the auto detection kicks in again.

And yes, you are right, the top comment should mention that i18next already does an automatic language selection based on the browser. I am pretty sure, that browser basically always provide this information, I'm not even sure a normal user could configure popular browser in a way to break this.

I personally assume that the auto language works perfectly for >99% of users. The remaining ones use the language menu to set one specific language. I doubt that any real user ever wanted to have this "auto" setting in the language menu. What for? Unlike a color scheme, which could conceivably change regularly (e.g. day/night with light/dark scheme), I don't see any situation where the browser language changes regularly.
So in my mind this issue is very very very low priority. Almost to the point of closing it.