Hi, I noticed the langage switcher in the header today, this translates the site, interface and content, to another language, but does this means I can post content in that other language or should I continue to post in English ?
You can use supported languages.
My opinion is that we should use English if possible. But not everyone can English, or are shy to use it because skills are close to my rally English [1], and then this option lowers language barrier big time. I really like this option, in here and generally.
Finnish rally drivers are known about heavy accent and using Finnish structure in sentences ↩︎
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Okay, but let’s imagine that I reply or post in French. If users who use the English interface do not activate multilingual translation, they will not be able to read my messages in French and may not even know that it is French?
Yes it is.
But if someone want to read in original language, then it is quote hard to get tranlated content 
It is matter of choice, from both parties. If you write in English, it is still translated to French. If you use English, it is translated to English. And my choise is choose between original and translated version — and it is quite easy choice, because I can about 50 words in French…
With Chinese that helps, a lot.
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Oh, did you mean this:
I changed to French, and that post was still shown in English. When I didn’t use localization, aka. translating, everything was in English. Did I mess something 
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Yes there seems to be a mix of languages in that case.
Another question, if people can write in any supported languages, when I browse the site in English, will I see those original languages not translated because English is the default language or will I see those different languages ? In other words, selecting English translates everything in English or leaves the content untranslated ?
It depends. There is a toggle near the timeline which allows you to decide whether to see everything in the language it was written in or to translate everything into your interface language. That works for all languages you can select in the header.
So for example in this topic there is a mix of German and English. My interface language on Meta is English. I can read the posts in the language they were written in or read everything in English.
I prefer to post in English because automatic translations are not perfect, and I like to control the version most users will read (or can at least use as a fallback when the automatic translation seems strange). Things get complicated when setting names are translated or when translators use slightly different terms when they translate the Discourse interface. For example in a German post I’d use “Team” for staff because that’s the name of the group when your interface is German. But AI will translate “Team” without that knowledge and there is no default “team” group in English. So it could easily be mixed up with the team group here at Meta.
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Yes though impressive I must admit, this poses some good questions.
Ich arbeite auch gerade an einer Community mit mehreren Sprachen, das Feature funktioniert eingetllich sehr gut. Nur es gibt so einige Sachen wie zerstörte Zitate oder auch das die AI Zusammenfassungen nicht übersetzt werden. Ansonsten bin ich zufrieden.
Edit:
Was mir ausgefallen ist das im Language switcher and language content - #9 by Moin in der Übersetzen Version das Video und Zitat nicht angezeigt wird.
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Actually everything after the video is missing. I’ll fix the German translation